also my 2nd name ends up like
"13:52:32 Closing Link: dhcp-077-249-139-114.chello.nl (Invalid username [_Aisaka_Ta])
13:52:32 *  Host disconnected"
witch suppose to be _Aisaka_Taiga_, so it must be a bug
bcause "`Aisaka_Taiga" works
The username _Aisaka_Taiga_ simply is truncated to _Aisaka_Ta. Every network has a limit on the length of nicknames and usernames.
If "`Aisaka_Taiga" does work, why not use it? The IRC network
prevents you from connecting with the username that starts with the underscore - it's not a bug, but their policy. Put on a plain nickname/username, connect to the IRC network and ask the staff at #help about the why.
My guess would be they disallow certain chars at certain positions of nicknames/usernames to keep some known spam-/floodbots off their network - but thats a guess only.
its not the network because they also accepted vhost and registration and im not the onlyone that use a name like this
Being able to do a thing one day isn't a warranty for all the future days. Also note that the different rules may apply to connecting clients and clients that already are connected. Again, only the IRC staff can know - go and ask!