This is definitely the entries table (as I assumed). There was the column email_notification up to the version 2.4.13. With the new structure to store subscriptions, introduced with 2.4.14 the column got removed but it still existed in the backup script. It is there only in the INSERT-statements for the restore that does not contain a column list. Default behaviour of MySQL in this case is to try to distribute the given data to all of the columns but the number of provided data columns in the INSERT-statement does not fit with the real number of table columns (valid for version installed before 2.4.14 and updated to version 2.4.14 or higher).

Additionally I found the column email_notification in the installation script. If the forum was installed with a newer version than 2.4.13 the column was created but never used. If one created a backup and restored it, the number of columns in the statements and in the table matched. With a version that was installed before 2.4.14 and where the update script removed the column with the update to 2.4.14 the number of columns does not match in case of restoring a backup.

Thank you very much for this professional work!

In my case I've installed my little forum version 2.4.21 (I did not upgrade from older version), so maybe in my case the problem is different?

Thank you, shian