TL;DR: There was a Steam bug and I fixed it locally.

Some months ago, Steam had the problem, that if you tried to add anything from the Steam Workshop to a collection, you would get an error like "Process failed: 2", while it was loading the collection list.
I realized, that it would work, but there was a bug in the JS (Watched the network tab in chrome while trying to add to collection). I searched after "Process failed" in each js file and after 30 seconds I found the buggy if. It said something like
if (json.success != 2) {
//do error
} else {
//show list
}

After I changed that if condition to
if (false)...
it worked perfectly, although it would make problems if there would be a server side error.