GCDS Sync Error - Unable to authenticate: Connection reset

[peterhorner]

We have a customer who have been using GCDS successfully for years but have now run into trouble. They intermittently receive the following message:

[2023-10-27 10:27:24,464+0100] [SwingWorker-pool-3-thread-4] [INFO] [sync.agent.FullSyncAgent] Applying computed changes to service.
[2023-10-27 10:27:24,593+0100] [SwingWorker-pool-3-thread-4] [ERROR] [plugin.google.OAuth2TicketVector] Unable to authenticate: Connection reset
[2023-10-27 10:27:24,594+0100] [SwingWorker-pool-3-thread-4] [ERROR] [plugin.google.GooglePlugin] Error while applying changes to remote directory.
com.google.usersyncapp.plugin.google.GoogleAuthenticationException: Unable to login
    at com.google.usersyncapp.plugin.google.OAuth2TicketVector.verifyApiConnection(OAuth2TicketVector.java:222) ~[DirSync.jar:?]
    at com.google.usersyncapp.plugin.google.OAuth2TicketVector.verify(OAuth2TicketVector.java:140) ~[DirSync.jar:?]
    ...

They've tried a different account to ensure it's authorised correctly. Additionally, they've checked firewall/network logs and there are no errors there. Google support advised to go back to a previous version but that hasn't helped.

Has anyone encountered this before?