Thank you for your help. Unfortunately, I was not able to sign in again. Ultimately registering a new account worked for me.
I suspect the issue is related to the use of an alias in the e-mail address (e.g. firstname.lastname@example.org instead of email@example.com). This problem might be prevented in the future if users are warned beforehand during registration to not use those.
Additionally, being able to edit the e-mail address one signed up with can also help resolve this problem without having to contact support/admins about this issue.
I hope you take these suggestions into consideration and thank you again for your help.