iMessage on Mojave Shows Ancient Chat History

I've updated both my iMac and my MacBook Air to Mojave. On the iMac iMessage is showing all my recent chats until today. On the MacBook Air the newest message shown is from July 2018. iMessage in the iCloud is activated on all my Apple devices, both of the Macs my iPhone and my iPad.

I've tried logging out and into iCloud and iMessage on the Air but with no success. Clicking the sync button in the iMessage settings does nothing. Also, when I uncheck the box "Activate iMessage in iCloud" I can then only click on "Deactivate All". "Deactivate This Device" does nothing, the window just keeps popping up again.