diff --git a/src/crypto/OlmDevice.ts b/src/crypto/OlmDevice.ts
index 2de8313d9..0b2e616a8 100644
--- a/src/crypto/OlmDevice.ts
+++ b/src/crypto/OlmDevice.ts
@@ -92,7 +92,7 @@ export interface InboundGroupSessionData {
     sharedHistory?: boolean;
 }
 
-interface IDecryptedGroupMessage {
+export interface IDecryptedGroupMessage {
     result: string;
     keysClaimed: Record<string, string>;
     senderKey: string;
@@ -100,6 +100,11 @@ interface IDecryptedGroupMessage {
     untrusted: boolean;
 }
 
+export interface IInboundSession {
+    payload: string;
+    session_id: string;
+}
+
 export interface IExportedDevice {
     pickleKey: string;
     pickledAccount: string;
@@ -620,7 +625,7 @@ export class OlmDevice {
         theirDeviceIdentityKey: string,
         messageType: number,
         ciphertext: string,
-    ): Promise<{ payload: string, session_id: string }> { // eslint-disable-line camelcase
+    ): Promise<IInboundSession> {
         if (messageType !== 0) {
             throw new Error("Need messageType == 0 to create inbound session");
         }
diff --git a/src/crypto/algorithms/megolm.ts b/src/crypto/algorithms/megolm.ts
index 8bf5a7c62..07636c11d 100644
--- a/src/crypto/algorithms/megolm.ts
+++ b/src/crypto/algorithms/megolm.ts
@@ -30,7 +30,7 @@ import {
     registerAlgorithm,
     UnknownDeviceError,
 } from "./base";
-import { WITHHELD_MESSAGES } from '../OlmDevice';
+import { IDecryptedGroupMessage, WITHHELD_MESSAGES } from '../OlmDevice';
 import { Room } from '../../models/room';
 import { DeviceInfo } from "../deviceinfo";
 import { IOlmSessionResult } from "../olmlib";
@@ -1280,7 +1280,7 @@ class MegolmDecryption extends DecryptionAlgorithm {
         // (fixes https://github.com/vector-im/element-web/issues/5001)
         this.addEventToPendingList(event);
 
-        let res;
+        let res: IDecryptedGroupMessage;
         try {
             res = await this.olmDevice.decryptGroupMessage(
                 event.getRoomId(), content.sender_key, content.session_id, content.ciphertext,
diff --git a/src/crypto/algorithms/olm.ts b/src/crypto/algorithms/olm.ts
index c640d14ef..e11a666e1 100644
--- a/src/crypto/algorithms/olm.ts
+++ b/src/crypto/algorithms/olm.ts
@@ -32,6 +32,7 @@ import {
 import { Room } from '../../models/room';
 import { MatrixEvent } from "../..";
 import { IEventDecryptionResult } from "../index";
+import { IInboundSession } from "../OlmDevice";
 
 const DeviceVerification = DeviceInfo.DeviceVerification;
 
@@ -331,7 +332,7 @@ class OlmDecryption extends DecryptionAlgorithm {
         // prekey message which doesn't match any existing sessions: make a new
         // session.
 
-        let res;
+        let res: IInboundSession;
         try {
             res = await this.olmDevice.createInboundSession(
                 theirDeviceIdentityKey, message.type, message.body,
diff --git a/src/crypto/olmlib.ts b/src/crypto/olmlib.ts
index 19c34fe2b..c017af081 100644
--- a/src/crypto/olmlib.ts
+++ b/src/crypto/olmlib.ts
@@ -323,7 +323,7 @@ export async function ensureOlmSessionsForDevices(
     }
 
     const oneTimeKeyAlgorithm = "signed_curve25519";
-    let res;
+    let res: IClaimOTKsResult;
     let taskDetail = `one-time keys for ${devicesWithoutSession.length} devices`;
     try {
         log.debug(`Claiming ${taskDetail}`);
diff --git a/src/models/event.ts b/src/models/event.ts
index 79e17d711..0ce12e068 100644
--- a/src/models/event.ts
+++ b/src/models/event.ts
@@ -823,18 +823,13 @@ export class MatrixEvent extends TypedEventEmitter<EmittedEvents, MatrixEventHan
                 //
                 if (this.retryDecryption) {
                     // decryption error, but we have a retry queued.
-                    logger.log(
-                        `Got error decrypting event (id=${this.getId()}: ` +
-                        `${e}), but retrying`,
-                    );
+                    logger.log(`Got error decrypting event (id=${this.getId()}: ${e.detailedString}), but retrying`, e);
                     continue;
                 }
 
                 // decryption error, no retries queued. Warn about the error and
                 // set it to m.bad.encrypted.
-                logger.warn(
-                    `Error decrypting event (id=${this.getId()}): ${e.detailedString}`,
-                );
+                logger.warn(`Got error decrypting event (id=${this.getId()}: ${e.detailedString})`, e);
 
                 res = this.badEncryptedMessage(e.message);
             }