This was reported while testing Beta 6.5 and it was marked as fixed. It's annoying that this is in again. This problem hits only people who update the board, not fresh installs. With a fresh install you cant create boards with _ inside the board name.
I encounter many trouble with old users convert from Classic. They have sometimes spaces in the username and I switch of special chars in threads. In result you have very stange effects with this users inside the board.
