I'm talking about the events in the Calendar.

I think you need the Calendar Tweaks add-on to make the background color the same as the category color (see picture).

https://addons.thunderbird.net/en-US/thunderbird/addon/calendar-tweaks/

https://support.mozilla.org/en-US/kb/installing-addon-thunderbird

The problem is gone. It started working this morning without doing anything...

Do you have the Tweaks add-on in Tools/Add-ons/Extensions? A restart may have prompted an update of the add-on to a 68-compatible version.

sfhowes said

Do you have the Tweaks add-on in Tools/Add-ons/Extensions? A restart may have prompted an update of the add-on to a 68-compatible version.

I have had the Extension running for a long time, and the colors were ok until something happened about a week ago. It could have been a W10 update. Not sure. But all is working now.

Thanks for your response.