Obviously, it's a Disconnection popup caused by network interruption. it's not even an error in the first place, "90k" is just a code.
I don't understand why people are complaining to SE about it, it's not their fault for FFS, it could possibly your ISP having problems connection to SE's network or vice versa, but I don't think it's the latter, as the players who seemed to be affected by this are the players who play from NA / EU. I rarely see anyone from JP data center or from SEA region who have this problem.