I've encountered a new problem that I'm not seeing online anywhere.
The Tl;Dr of the problem: in my GM mode game, after adding some new levels using premade content between play sessions, my players cannot use certain skills. For example, mosquito swarm does not work for anyone (they click on the icon and nothing at all happens).

The loner description:

What happened
-I'm running a GM mode game for 4 other players.
-I have two custom mods that I made myself to add a handful of levels and skills.
-We did a 7hr play session using these mods and everything was good, no issues at all.
-After the first long play session I added a bunch (17) of new encounters using the premade levels, so no custom mod stuff. I did not change the mods at all between sessions.
-When loading in for our next play sessions, the players found that they could not use certain abilities, such as mosquito swarm or sucker punch. When clicking the icons, nothing at all happens, no animation or error message.

Debug attempts tried
-We double checked that everyone had the mods checked, and that everyone had the proper skills needed to use the abilities, everything is set up right.
-We closed out of the game and reopened it, no luck.
-We ALL validated our game files, no change.
-We loaded into different levels, no change.
-I removed the skill known from a player and gave it to them again from my GM menu, no change. I even removed it and gave them a new skillbook, no change.
-We rerolled the characters from within the game, no change.
-The players made new profiles and loaded back in, no change.
-We checked if it mattered if the skill came from an item or was learned: it doesn't matter, both item skills and learned skills are broken.
-I gave them a scroll to cast from, this actually gave the starting animation, but they still couldn't cast the skill.
-I checked if NPCs could cast the spells: NPCs CAN cast the spells fine with no issue!
-I tried to rebuild my mods but there was no change.
-Another player hosted a new GM mode game and we could use skills just fine in their new game.

Summary of debug
It seems like something is broken for the players themselves, since the skills work on NPCs. It doesn't really matter the origin of the skill, the skill just won't work. It also appears to be that the same skills are broken for everyone, it's not on a character-by-character basis.

I've not seen this problem online, and I've modded a lot in the past and never encountered it myself either. Does anybody have any ideas? Hopefully people are still looking at DoS2...