I cannot find any solution to the problem I am having so far. I hope someone can help me.
I have a site collection with a custom master page. When I edit it (change the default css file used) I am getting a "Code blocks are not allowed in this file." error when opening a document set in a document library. The rest of the site (collection) works fine.
Not a problem, I though. I just roll back the change, and restore the previous version of the master page. Publish, approve, etc., but guess what? The error remains. Fortunately, I still had another copy present of the master page which was already on the system since we first created our custom master page. I selected this one as master page on the site collection, problem solve.
Now I got curious....what if I make a copy of the master page currently selected - the one that gives no errors - and upload it under a different name. So master page xyz.master was giving no errors: 1. copy it 2. rename it to abc.master 3. let the site collection use this "new" master page.
Guess what? Errors again! So I am using an identical master page here, an exact copy of the master page that works fine, but still I am getting errors.
Has anyone a clue what's going on here?
Thanks so much for your answer.