The only other suggestion I have left is for you to go into the Tools/Options/Maintenance.
Use the CleanUP Now button.
This should clear out all downloaded messages from the email client.

Make copies of whatever is important, or copy them to a folder OUTSIDE of the email client.

You may also wish to check “Delete msgs in the Delete
folder upon exit”

Were you at all successful in attempting to delete the message after starting in Safe mode?

Techie