Falls jemand hier ähnliche Probleme hat:

Please ensure the Apple Mail plugin is correctly installed and macOS permissions given.
Quit Mail.
Quit Pro Office.
Enable Full disk access in System Preferences > Security & Privacy for DEVONthink Pro Office
In the Finder, hold the Option key and choose Go > Library.
Go into the Mail/Bundles folder and delete the DEVONthink mail conduit bundle.
Relaunch Pro Office and install the Mail plugin from the DEVONthink Pro Office menu item > Install Add-Ons.
Relaunch Mail and enable the plugin via Apple Mail's Preferences > General > Manage Plug-ins.

PS: The current version of DEVONthink 2.x is 2.11.3 and is available here: https://www.devontechnologies.com/support/download/legacy