GitLab server issue: Two-Factor Authentication gives error in time synchronisation (?!) [FIXED]

Today I logged in to the GitLab server for ISPConfig, and, to my surprise, it didn't accept my TFA TOTP code. It was a bit weird, and the error — after looking it up — pointed to a time synchronisation issue.
I immediately suspected that my TOTP application (Duo Mobile) was the culprit, so I first logged in with one of the "emergency keys" I had (it worked flawlessly). Then I removed the entry for this GitLab server on the app and tried to add it again via QR code.
It certainly created the new entry, but, alas, I couldn't confirm the code — the error was the same.
Ok, so I deleted everything again and this time tried it out with Google Authenticator. To my complete surprise, it didn't work either! Same error: time sync issues.
Now I'm pretty sure that it's not a problem with my iPhone and/or my Mac — both are definitely being updated via NTP — so I wonder if it's somehow related to the maintenance made a while ago? I think I didn't log in since then.
I actually just wanted to add a YubiKey as an alternative TFA, but it seems that you must have TOTP TFA set up first, and then you can pair with your key — but it doesn't work the other way round (why, I don't know; possibly because if both your TOTP app and the YubiKey are broken/missing, you'd still be able to log in with one of the emergency keys — which requires setting up TOTP TFA first...).
I wonder if there is something I can do to fix this? Or perhaps just try later?
Has anyone else had any issues with TFA to the GitLab server recently, or is it just me?
For now, of course, I've turned TFA off, but I certainly would prefer to have it on...

 

... I should have mentioned that I have 140+ TOTP entries on that app and all of them are working. Well, at least most of them, I didn't try them all, one by one! But I certainly have tried out many (including the GitLab public server).

 

Thanks, @Th0m

 

Awesome! That fixed it for me, and I could add the YubiKey as well

Thanks so much!