"The operation failed. The messaging interfaces have returned an unknown error" in Outlook

[Marios Petrou]

I'm having this error on some client's Outlook 2016 on laptop/PC.

This only seems to happen when they leave an email/draft open for a few hours. Closing and reopening Outlook works, but they lose their work.

Already tried recreating Outlook profile, installing all pending Office/Windows updates, repairing Office, but the issue remains.

Any ideas?

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