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I bought White March P1 and before installing it i had to update all the way fom Patch GOG 2. After White March i installed Patch 2.00-->2.01.

I start the game and receive the letter for the new Region and the Sidequest northeast of Caed Dun. I proceed to the latter and after loading the Level is heavily bugged: It lags, the snow particles are missing textures(they are in plain purple placeholder), the mercenarys are static, but i am able to attack them though they cant die.

My question is: Is that a feature?
I think you need to install the white march specific 2.01 patch rather than the generic 2.01 patch. You will have to reinstall or import the game into Galaxy so it can take over and fix it.
how about gog fixes that?!

what happened to:
muh drm free
muh no client
muh standalone
muh customer service

i am starting to feel the presence of gaben here .it all began with the announcement of the witcher 3 PREORDER DLC at first day of release, today i have to witness geralt has been painted blue for 8 €.
Not use what are you expecting. You used the wrong patch file that is missing the DLC content, use the correct one listed in the games's page. I heard from someone that made the same mistake as you did that using that over the game will fix the problem, no need for a reinstall after all.