"Non-resolvable dependency" - missing version number?

Thanks for the issue @dgwolf I will take a look within the next day or two. If the modules actually updated via composer, you are probably good to go, but the lingering message is something I want to resolve.

Thanks, very good.

Does this still happen after the upgrade to 3.1.5 and 3.4.1? Can you send a screenshot or the original native language text if yes.

After the upgrade with composer I get the same message in German:
"Nicht auflösbare Abhängigkeit
Registration Wait List (Version >=3.1.2 erforderlich)
Commerce Registration Wait List benötigt dieses Modul und Version. Aktuell wird Registration Wait List Version verwendet."

When entering text from the last line at admin/config/regional/translate I see the orignal English messages would be

Unresolved dependency
@required_name (Version @compatibility required)
@name requires this module and version. Currently using @required_name version @version

- so it seems that @version is an empty string that gets compared to @compatibility. This seems to trigger the error message that as @version can subsequently only display the empty string.

You must have the registration module checked out, instead of it being installed through composer. When you check the module out from GitLab, the version info is not appended to the .info file, so Drupal can't find the version number. Move the checked out module elsewhere (in case you have any changes or stash) and then do a composer install and you should be good to go.

Thanks a lot, John, that was it.

I cannot remember having checked out consciously from GitLab but I did try out one of the dev versions some time ago, suppose this came from GitLab.