Hi there.
I hope it’s okay I write in English here, and I’m sure @OttoKerner can assist with translation if not 
First of all, if you simply deleted your account, then your QNAP PMS doesn’t know about this, and is still trying to logon with your old account.
As such, you need to manually fix your preferences.xml file
Take a peak here: https://support.plex.tv/articles/204281528-why-am-i-locked-out-of-server-settings-and-how-do-i-get-in/
(And a QNAP is a Linux box)
The most easy way to access your preferences.xml file, is to install this:
Above will create a share named Plex, where you can find your preferences.xml file in the Plex Media Server directory
After that, restart your PMS
Then when browsing, you should see a dialog about an unclaimed server, and if not, then use this little gem:
