It's either this mod or Interstellar Incorporated which relies on this, but when both of them are enabled, my fleet power (as well as the AI's) skyrockets - my starting 3 corvettes, for example, have 4.3k fleet power. Not only this, but neither my ships nor hostile AI ships will shoot each other when they get into range with each other. Not sure what it could possibly be conflicting with, but here's my modlist:

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2073131252

SLEX doesn't change anything on its own. I've been looking a bit at your collection, but nothing catched my eye.

For the fleet power bug, I have really no idea as neither IISCO nor SLEX change anything (to my knowledge) to the fleets of the "default" countries.

For the shooting problem, if the fleet can't actually take a hostile action against each other, that may be because IISCO overwrites the game rule "can_take_hostile_actions" and if another of your mods do the same, there might be an issue.
If the fleets are in a fight, but don't actually shoot, then either it's a vanilla problem (happens sometimes, but usually it's just one of the 2 fleets that won't shoot), or it's a range issue. Or it is something else.

TL;DR I don't really know except it's not coming from SLEX for sure, it's very likely not intentionnaly coming from IISCO, so it's probably a compatibility problem. But I don't know why the fleet power would be affected by IISCO in the first place.

Yeah, I've got genuinely no idea why this has happened. There must just be some sort of incompatibility between IISCO and something else I've got on, no real idea what it could be though. Although the weird thing is that I never had this issue immediately after installing IISCO and SLEX, it just broke overnight a few decades into a fresh save pretty much.

Oh, so it was probably caused by a mod (possibly IISCO) updating a bit after you initially installed SLEX and IISCO.
You can quickly check the date of last update of your mods by looking into "SteamGames\steamapps\workshop\content\281990"
Every number is the id of a mod and the "Last modified" would indicate when you downloaded it.

So mods that weren't updated after you installed SLEX (mod id : 2054717582) are (probably) not the cause of the problem. (As SLEX wasn't updated in a while, the mod updated after it are mods that were updated after you initially installed IISCO and SLEX).

The only mods that updated after I installed SLEX are Extra Ship Components and the Machine Shipset it appears. Dunno how those could really make the game stop working though since the latter is cosmetic and the former doesn't actually touch any early tier weapons/components (as far as I know)

I checked the possible overwrites between SLEX, IISCO, ESC and the machine shipsets and there is nothing. And both of them don't seem to alter vanilla "early ships" in any way. So I really don't know where the problem is coming from :(

It seems the issue has come back and I haven't reenabled IISCO or SLEX, even though it initially disappeared after I removed them. I found out that the weapons range has been reduced to 0, which explains the ships being unable to shoot, however their damage per tick is 1000, which explains their increased fleet power. I'm guessing ESC's update today has broken something, I dunno.

Can you see where the debuff is coming from ? Maybe by typing "debugtooltip" in the console ?
There might be some additionnal text when you hover on the modifier or on the ships. It could suggest the name of the responsible mod (hopefully)

Using debugtooltip, there's no actual debuffs.. armour, shield, and hull HP are all buffed 200% (despite me not having any civics or ethics or anything else buffing like that). Also, my ships now start with small amoeba weapons which... makes no sense at all, I have no idea why.

I already tried verifying my game cache twice, and it hasn't fixed this. I'll try just deleting all the UGC files in the Stellaris mod folder and restarting the game, pretty sure it should redownload them. If not I'll just resub to all the mods in my collection lol. Will report back.

Well, looks like my combination of mods just suddenly stopped playing nice, possibly related to that ESC update earlier.