Unable to register eM Client as Standard Email App

On a freshly installed Windows Server 2019 Standard Server I am unable to set Em Client as the standard Mail client. During installation the Standard-App-Window appears but I can’t select Em Client because is is not selectable in the Windows Standard App section.

It is possble to select some protocol assignments to eM client, such as .eml, .ics, .msg and .vcf but not for mailto, webcal or webcals.

Interestingly eM client works without problems, even sending emails from other application via MAPI works with some applications, but not with all.

As this problem does not appear on other Windows Server 2019 installations with identical up to date patch level, it does not seem to be an incompatibility issue per se but I assume that Microsoft has changed some security setting on freshly installed OS environments that prevents eM client to be correctly registered.

Any ideas?