diff --git a/spec/integ/matrix-client-syncing-errors.spec.ts b/spec/integ/matrix-client-syncing-errors.spec.ts
new file mode 100644
index 000000000..ab11a33ad
--- /dev/null
+++ b/spec/integ/matrix-client-syncing-errors.spec.ts
@@ -0,0 +1,162 @@
+/*
+Copyright 2023 Holi Moli GmbH
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import "fake-indexeddb/auto";
+import fetchMock from "fetch-mock-jest";
+
+import { MatrixClient, ClientEvent, createClient, SyncState } from "../../src";
+
+const makeQueryablePromise = <T = void>(promise: Promise<T>) => {
+    let resolved = false;
+    let rejected = false;
+
+    // Observe the promise, saving the fulfillment in a closure scope.
+    const newPromise = promise.then(
+        (value) => {
+            resolved = true;
+            return value;
+        },
+        (error) => {
+            rejected = true;
+            throw error;
+        },
+    );
+    const isFulfilled = () => {
+        return resolved || rejected;
+    };
+    const isResolved = () => {
+        return resolved;
+    };
+    const isRejected = () => {
+        return rejected;
+    };
+    return { promise: newPromise, isFulfilled, isResolved, isRejected };
+};
+
+const queryablePromise = <T = void>() => {
+    let resolve!: (value: T | PromiseLike<T>) => void;
+    let reject!: (reason?: any) => void;
+
+    const promise = makeQueryablePromise<T>(
+        new Promise<T>((_resolve, _reject) => {
+            resolve = _resolve;
+            reject = _reject;
+        }),
+    );
+
+    return { resolve, reject, ...promise };
+};
+
+describe("MatrixClient syncing errors", () => {
+    const selfUserId = "@alice:localhost";
+    const selfAccessToken = "aseukfgwef";
+    const unknownTokenErrorData = {
+        status: 401,
+        body: {
+            errcode: "M_UNKNOWN_TOKEN",
+            error: "Invalid access token passed.",
+            soft_logout: false,
+        },
+    };
+    let client: MatrixClient | undefined;
+
+    beforeEach(() => {
+        client = createClient({
+            baseUrl: "http://tocal.test.server",
+            userId: selfUserId,
+            accessToken: selfAccessToken,
+            deviceId: "myDevice",
+        });
+    });
+
+    it("should retry, until errors are solved.", async () => {
+        jest.useFakeTimers();
+        fetchMock.config.overwriteRoutes = false;
+        fetchMock
+            .getOnce("end:versions", {}) // first version check without credentials needs to succeed
+            .getOnce("end:versions", 429) // second version check fails with 429 triggering another retry
+            .get("end:versions", {}) // further version checks succeed
+            .getOnce("end:pushrules/", 429) // first pushrules check fails starting retry
+            .get("end:pushrules/", {}) // further pushrules check succeed
+            .catch({}); // all other calls succeed
+
+        const syncEvents = Array.from({ length: 5 }, queryablePromise<SyncState>);
+
+        client!.on(ClientEvent.Sync, (state: SyncState, lastState: SyncState | null) => {
+            let i = 0;
+            for (; i < syncEvents.length && syncEvents[i].isFulfilled(); i++) {
+                // find index of first unfulfilled promise
+            }
+            syncEvents[i].resolve(state);
+        });
+
+        await client!.startClient();
+        expect(await syncEvents[0].promise).toBe(SyncState.Error);
+        jest.runAllTimers(); // this will skip forward to trigger the keepAlive/sync
+        expect(await syncEvents[1].promise).toBe(SyncState.Error);
+        jest.runAllTimers(); // this will skip forward to trigger the keepAlive/sync
+        expect(await syncEvents[2].promise).toBe(SyncState.Prepared);
+        jest.runAllTimers(); // this will skip forward to trigger the keepAlive/sync
+        expect(await syncEvents[3].promise).toBe(SyncState.Syncing);
+        jest.runAllTimers(); // this will skip forward to trigger the keepAlive/sync
+        expect(await syncEvents[4].promise).toBe(SyncState.Syncing);
+    });
+
+    it("should stop sync keep alive when client is stopped.", async () => {
+        jest.useFakeTimers();
+        fetchMock.config.overwriteRoutes = false;
+        fetchMock
+            .getOnce("end:versions", {}) // first version check without credentials needs to succeed
+            .get("end:versions", unknownTokenErrorData) // further version checks fails with 401
+            .get("end:pushrules/", 401) // fails with 401 without an error. This does happen in practice e.g. with Synapse
+            .post("end:logout", unknownTokenErrorData) // just to keep up a consistent scenario. Does not have a real effect for this testcase
+            .post("end:filter", 401); // just to keep up a consistent scenario. Does not have a real effect for this testcase
+
+        const firstSyncEvent = queryablePromise<SyncState>();
+        const secondSyncEvent = queryablePromise<SyncState>();
+        client!.on(ClientEvent.Sync, (state: SyncState, lastState: SyncState | null) => {
+            if (firstSyncEvent.isFulfilled()) secondSyncEvent.resolve(state);
+            firstSyncEvent.resolve(state);
+        });
+
+        await client!.startClient();
+        const logoutDone = queryablePromise();
+        client!
+            .logout(true)
+            .then(() => {
+                logoutDone.resolve();
+            })
+            .catch((e) => {
+                logoutDone.resolve();
+            });
+
+        const syntState = await firstSyncEvent.promise;
+        expect(syntState).toBe(SyncState.Error);
+        jest.runAllTimers(); // this will skip forward to trigger the keepAlive
+
+        jest.useRealTimers(); // we need real timer for the setTimout below to work
+
+        const timeoutPromise = makeQueryablePromise(new Promise<void>((res) => setTimeout(res, 1)));
+
+        await Promise.race([secondSyncEvent.promise, timeoutPromise.promise]);
+        // when syncing stopped, then the secondSyncEvent will never happen and the promise will not be resolved,
+        /// so the timeoutPromise will be resolved instead
+        expect(timeoutPromise.isFulfilled()).toBe(true);
+        expect(secondSyncEvent.isFulfilled()).toBe(false);
+
+        await logoutDone.promise; // wait for the logout to finish to prevent processing and logging after the test is done.
+    });
+});
diff --git a/src/client.ts b/src/client.ts
index 69f21560b..32522d330 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -1552,7 +1552,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
             this.clientOpts.threadSupport = this.clientOpts.experimentalThreadSupport;
         }
 
-        this.syncApi.sync();
+        this.syncApi.sync().catch((e) => logger.info("Sync startup aborted with an error:", e));
 
         if (this.clientOpts.clientWellKnownPollPeriod !== undefined) {
             this.clientWellKnownIntervalID = setInterval(() => {
diff --git a/src/sync.ts b/src/sync.ts
index be13b0cce..84b625518 100644
--- a/src/sync.ts
+++ b/src/sync.ts
@@ -1604,6 +1604,17 @@ export class SyncApi {
      * @param connDidFail - True if a connectivity failure has been detected. Optional.
      */
     private pokeKeepAlive(connDidFail = false): void {
+        if (!this.running) {
+            // we are in a keepAlive, retrying to connect, but the syncronization
+            // was stopped, so we are stopping the retry.
+            clearTimeout(this.keepAliveTimer);
+            if (this.connectionReturnedDefer) {
+                this.connectionReturnedDefer.reject("SyncApi.stop() was called");
+                this.connectionReturnedDefer = undefined;
+            }
+            return;
+        }
+
         const success = (): void => {
             clearTimeout(this.keepAliveTimer);
             if (this.connectionReturnedDefer) {