So, managed to solve the issue.
I believe that what happened is related to this topic: 18.3 Studio not saving packages and other 18.3 specific errors - #4 by loginerror
Previously I have installed the custom-made packaged first, and later the Credentials package, so I uninstalled both, and installed Credentials first, and than the other. It worked, but it seems like it should not be this way.
Thanks for the help @loginerror