This appears to be an Owl problem introduced with v0.7.7.2. By now v0.7.7.4 is out. You may want to give it a try, and restart Thunderbird after updating the add-on. Please report back your results.

Thanks for the idea, but that didn't solve the issue

This is the advice I have received from the OWL people.

This was a known problem that appeared in the last few days. There is already an update available that fixes it for most people. Can you please install Owl version 0.7.7.4, restart Thunderbird (a restart is necessary for Owl), and see whether the problem persists? If the problem continues, please try to create a new Thunderbird profile, install Owl, and try to set up the account there. If it still doesn't work, please write to owl.support@beonex.com with a copy of your error console (Thunderbird menu | Tools | Developer | Error Console, and copy the entire contents as text), please.

Thanks for the tip, but that didn't fix the issue. I have reached out to owl support

Thanks for the tip, but that didn't fix the issue. I have reached out to owl support

Owl is now up to 0.7.7.5. "Hotfix for disappearing Inbox contents"

Thanks - just installed - let's see what happens