An issue does not get synchronized and appears in the troubleshooting section with the category 'Fallback user does not exist' and the message, e.g. 'The fallback user (username) for the user (other_user) does not exist'.
The fallback user which has been configured in a user field mapping does not exist (anymore) and thus cannot be selected as a fallback user.
Check your user field mappings (e.g. assignee, reporter or others) where you've used the fallback user and replace it with an existing user. If you did, the following steps should be done in order to resolve this problem:
- Change the fallback user for in your user mapping settings in the synchronization configuration.
- Perform a retry for the affected message(s).