Try to rebuild the index file of the troubled folder.

Right-click the folder - Properties - Repair Folder

Thanks. It solved the issue at the cost of producing hundreds of duplicates. The excellent add-on "Remove duplicate Messages" is currently incompatible with TB 143 and 144 but fortunately another one named "Duplication Remover" still works. Regards, Gérald

That is something of a worry. The 1.1.70 date is a clear sign of data corruption as is signifies the date stored as zero in the data file. It is a legacy of the c programming language. Dates start numerically on the first of January 1970.

THat you managed to fix that with a rebuild of the index indicates that the corruption was in the index file (MSF) used to populate the Thunderbird interface so it is not rummaging through gigabyte of data to show a simple list of current mail.

The appearance after the fix of loads of duplicate messages indicates you are getting multiple copies of the message you do receive as the repair trawl through the actual mail store to rebuild the index. The add-on clearly met your immediate need, but I think you have an ongoing issue is your account a pop mail account or IMAP? We are left with the question of Why? Do you have an antivirus product that might be interfering here on your Apple product?

Thanks. My account is pop. I have no antivirus interfering with email.