Unable to log in or reset password

After having created my account about two months ago and using dynv6 for some testing without any issues I noticed yesterday that I am unable to log into the web interface. I was unable to do API testing because I would need my passphrases/keys that are only accessible via the website.
After typing the correct email and password I get an “invalid email or password” error.
When trying to reset my password it tells me my email address was “not found”.
But I am also unable to create a new account with that same mail address because it is marked as “is invalid” and the zones are still taken (tested with this “replacement” account).
I confirmed my original account after creation and still have the email, so it’s not like I am using the wrong mail address.
I found a thread about a “reconfirmation email” being marked as spam

but I didn’t receive anything in any folder (including spam) of my Gmail account…
The only other explanation I could come up with is getting banned, but one ddclient request every 15 minutes, without any --force parameters or similar (I got no warnings about “abusive behaviour” in verbose mode, just a simple “update skipped”) seems reasonable.
I also didn’t use the service in the last week, and while I was using it I was still able to log in. So a ban also seems unlikely to me especially because I was not even using the service before noticing the irregularity.
Any help for either getting my account back or freeing up the zones (I don’t want to post my email or zones publicly, if it’s not necessary but can share them at request) would be appreciated.
Communication in German is also possible.