Default E-Mail is disabled when sending a document per Mail

[MoonKid]

I have a pdf formular (not build by myself) then can send itself by mail.

The problem is that the default e-mail application is disabled as you can see in that dialog screenshot ("Standard-E-Mail-Anwendung").

It is Windows 10. I checked the "Standard Apps" settings of windows. I tried different settings there for "E-Mail": "Mail", "Thunderbird", "claws-mail.exe". I restarted my Adobe Reader DC 2019.012.20034 between each try.