The add-on author still needs to update it to be compatible with TB91.

Well tks for news. Disturbing that TB gives information in addon-selection "compatible with all releases..."

The latest add-on version currently available on AMO is compatible up to TB78.* https://addons.thunderbird.net/en-US/thunderbird/addon/importexporttools-ng/

If you want to backup your emails, use the free application Mailstore till import export tools by is updated for use in TB91.

Tks a lot to Lampros, that helps inbetween.... Regards Horst

They are making good progress for version 91. Betas available at https://github.com/thundernest/import-export-tools-ng/issues/264

Download the zip file, change the file extension to XPI, and use tools > add-ons > install from file