Updated FAQ

FAQ.md

@@ -29,6 +29,7 @@ For authorizing:
 * "*... Couldn't read row ...*" causes sometimes a crash. This could be caused by a bug in the [Room Persistence Library](https://developer.android.com/topic/libraries/architecture/room) but more likely indicates a corrupt database.
 * A [bug in Android](https://issuetracker.google.com/issues/119872129) "*... Bad notification posted ...*" lets FairEmail crash on some devices after updating FairEmail and tapping on a notification.
 * A [bug in Android](https://issuetracker.google.com/issues/62427912) "*... ActivityRecord not found for ...*" sometimes causes a crash after updating FairEmail.
+* Encryption with a token like [YubiKey](https://www.yubico.com/) results into an infinite loop. FairEmail follows the latest version of the [OpenKeychain API](https://github.com/open-keychain/openpgp-api), so this is likely being caused by an external bug.
 
 ## Planned features
 
@@ -391,6 +392,8 @@ If you want, you can verify a signature by opening the *signature.asc* attachmen
 
 Encryption/decryption is a pro feature.
 
+Please see the [known problems](#known-problems) for a problem with tokens like a YubiKey.
+
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