Synchronization is blocked if attachments are synced to a Windows Server.
If you use a Windows Server to host your Jira on-premise instance, then the problem might be caused by the special characters in the Connection name.
Windows has a set of characters that are not allowed for file names in the system. More details.
Update the Connection name via database query on both sides:
For Jira Server and HP ALM/QC
- XXX is the name of the Connection
- YYY the Connection ID