The jooa11y Button shows 87 errors on an empty edit form.

Your screenshot is a but misleading as those yellow ? icons in the image are warnings and not errors. They are valid warnings as they simply say "Jooa11y cant check this but I think you should"

The errors are all in the calendar popup and are valid errors that should be fixed in the calendar. The same accessibility errors are present in the admin wherever the calendar is used.

In summary it is not something to fix in the jooa11y checker but it is something to fix in the calendar widget

Correct, these are warnings, not errors, sorry. But this looks like 87 errors:

Normal users who only want to edit an article in frontend don't understand that there are warnings where they cannot do anything.
Additionally these Icons make working with the form difficult. Example - enter a tag here.

Maybe we can exclude Forms in Frontend from beeing scanned by the plugin.

Please read more carefully what I wrote

I am in a course for editors who have to write articles in frontend. They don't understand these warnings at all. I did not say that it is an error of the jooa11y plugin, but the problem occours when the plugin is activated.

In the config of the plugin you can set areas to ignore. In this case set it to form

thanks, that works.