Google Tag Manager Tag for custom script is not firing even though trigger has fired

The Problem is pretty much in the title.
I have been trying to do a simple script insertion for a customer of mine for a while now, but the tag is not firing, even though the trigger has been fired, there are not blocking triggers and the firing options are set to once per event.
As far as i can tell from the Google Tag Deployment Rectriction List that’s not the problem either. The website explicitly whitelists “google” and “img” and only blacklists “adm”, “sp”, “gcs” and “opt”, which shouldn’t interfere with my script tag as far as I’m aware.

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I have already tried a number of different ways to trigger the tag and also tried switching up the firing options but it still refuses to fire and implement the script.
When I try implementing another tag type it works, so there HAS to be a problem with the script tag and the blacklist, but I can’t figure out what exactly is blocking the script tag.