Bewertet mit 5 von 5 Sternen

There's something wrong with affixes. It's not recognising simple things like cats or generated.

Edit: Updating to 60.5 fixed it

Diese Bewertung wurde für eine vorherige Version des Add-ons (2.69) abgegeben. 

I know.

V2.69 of the GB speller is now a WebExtension and it requires at least Thunderbird 60.5 (29-JAN-2019).

Mozilla triggered the updates of the speller faster than the trigger of Thunderbird.

Go to the menu: Help > About, and update from Thunderbird 60.4 to 60.5 and it should work.

Please tell me if it fixes the issue.