Pit I would recommend you ask your hosting about this. Surely they should notify you if they are going to block an application? And block your entire domain? Thats kinda preposterous from a paid service.
There are a couple earlier posts about similar issues, which have been resolved. I can't solve issues with hosts that want to block an application without any legitimate explanation. Files Gallery includes a file manager, and maybe some hosts get paranoid about that, but surely it's one of the reasons the user wants to use it in the first place? There is after all login protection.
This users host first blacklisted the file (for no valid reason), and then whitelisted it later on.
https://forum.files.gallery/d/643-malicious-exploit-infected-file
In this case, Kaspersky anti-virus had detected Files Gallery as a malicious file, something they corrected later on when they discovered it was a false detection.
https://forum.files.gallery/d/748-security-issue
Since Files Gallery index.php is a single file, it's quite simple for a programmer to review the code and see there is nothing malicious in the file.
https://github.com/mjau-mjau/files.photo.gallery/blob/master/index.php
Of course, there are file manager capabilities, which would cause problems if accessible without login, but it should be the owners right (and responsibility) to setup this.
You should ask the hosting for an explanation. If they say the file is "malicious" or contains "malware", then you would need to ask from where they get that, something they won't be able to answer. May I ask who this hosting service is?