I posted this Problem on the SW-Subreddit and got the recommendation to post it here so @TI can see it (you were officially declared "PDM genius")

Pre-story: I ran into a quite interesting problem. I got "handed over" (aka got the admin login and almost no documentation) a PDM at the new company I work at, and I did some needed cleanup or "making things right". Formerly all users were in one "construction" group, and permissions were set per user. I corrected that, set up groups and everything looked fine at first

A colleague, who I moved to a new user group suddenly could not see certain files. But strangely this was neither status nor folder related, he would see half of the files in a folder but without any obvious pattern. All the permissions were set correctly.

After way too much time I found out that someone had set up one colleague (who does not work here anymore) with "permissions by file" in a way that only the "construction" group could see files he created. I changed the setting, but this permission is set on file level (so it persists).

Question: How can I modify hundreds, maybe thousands of file properties (not a "normal" file property that can be changed via data card or workflow) at once on the PDM? The Option is greyed out as soon as I select more than one file.