I have experienced this bug, and as far as I know, there is no way to fix it after it occured, except buying the White March DLC - For those who don't know, the bug is caused by the auto-leveling of Hiravias, the GM and Durance past a certain level, because the auto-leveling tries to had capacities from the DLC.
What worked for me is loading a save before I recruited Hiravias (the character that caused the bug for me), disable Auto-leveling, and then recruit Hiravias and go on. Having done that, no problem at all, neither with changing the party nor accessing the first floor of Brighthollow.
BUT, if you don't have a save before recruting... Sorry, but it's dead for you. After some tries, before I reload my save before Hiravias, by changing my party to five members on the screen in the stronghold to match the party I had in game (Me, Eder, Durance, Pellegina, Aloth), saving, closing the game and reloading, I manage to be able to had Kana back to my party, obtaining a six functionning members party which could have allowed me to continu the game, only preventing me to acces Hiravias and the GM. But there is no real fix for the bug.
I suppose Obsidian is trying to work a way for the auto-leveling to detect if you have the WM DLC and adjust the auto-leveling to your version of the game, allowing you to use the auto-leveling even if you have the DLC without losing acces to interesting capacities... But for those of us who play the vanila version... It's a real problem, and should be fixed.
