Thank you!
Unfortunately the FTP access still does not work, I can only access an empty folder.
I was able to log-in though, and located the problematic script. I disabled that file via the plugin editor on the back-end. It is not a crucial script, everything will work just fine now.
Still, if possible, I would like to debug this furhter, as I could not locate what code part is actually causing the conflict, and without being able to edit the files via FTP I may not be able to do so. If not possible, then I would like to try via the plugin editor, but it may take a lot more time.