Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.

I restarted Thunderbird in safe-mode with add-ons disabled.

The CPU has dropped to 5-15% usage. I had only 3 add-ons ... BUT the Warnings and errors keep coming in steady - just like before starting in safe-mode ... see attached image.

I am getting "Uncought exceptions ..." and other Errors as well ...

Just tried to repair an inbox folder (the one I suspect to have problems) ... Still running in Safe mode ...

Then all these errors came forward.

Can it be so that the database for the inbox is corrupted somehow ...?

The warnings and errors are basically normal, the mso ones are basically things in the body of the email being rendered. The deprecation of non UNICODE is also really a standard warning and again almost certainly coming from the mail in your mail box.

I would suggest opening your profile folder and deleting the file GLobal-messages.db.sqlite while Thunderbird is closed and let it regenerate.

Thank you Matt, The global-messages-db.sqlite was deleted (320Mb) and Thunderbird generated a new database, but this time it reached only 159Mb., so something must have been wrong here ...

I deleted all add-ons and reinstalled them.

Now, I am down to some 0-5% CPU and no spinning activity-wheel anymore.

Thanks!