We're playing FTB Infinity Evolved Expert and we added ars magica 2.
Ars magica 2 has mage robes that can prevent you from being set on fire. There's also a burnout bar when you use magic that fills up and has other mechanics like increasing mana cost and whatnot.
The issue comes in when I fill this burnout bar. A flame icon appears near my crosshair and I'm set on fire. Note that this isn't a mechanic in ars magica 2. Filling my burnout should NOT set me on fire, ever, for any reason. Which makes this confusing.
The leggings of these mage robes try to put out the fire and BAM! It crashes my game and anyone else who happens to be nearby. It continues to crash with the same report every time I join the server until my effects are cleared. When my effects are cleared I'm usually at half a heart and still on fire. The crash report mentions blood arsenal every time. I am the only one this whole fire shenanigan happens to as well. It started only when I first began to wear the mage armor.

I do doubt it has to do with your mod, but since it's in the crash report I am here.
Here's the report: https://pastebin.com/FsVU2JDZ

I am also having this problem.

I am also having this problem.

Turns out this issue is caused by conflicting potion IDs. Change Ars Magica or Blood Arsenal's potion IDs in their respective configs to something that isn't taken and you should be good to go. You may have to install a potion ID extension mod so they'll actually fit in with all the potions that are in infinity evolved(unless it's already in there, I can't remember so check).