Ok, that explains a lot. Although most of them are 3-letters-bad-words that could be anywhere in the middle of the most common names...
The "profane" names were "Shizuru" and "Kukuri", I even tried to combine numbers and other letters but I kept receiving this error. I would never expect them to block even non-English bad words.
I also sent a 1:1 about this and I got this answer:
Maybe someone really is using the name or they just don't want to change the language filter.
Well, whatever, I finally found an "usable" character name.
Thank you all for your help.