Hey Shat, how are you?
Unfortunately, your image wasn't attached, so I made some assumptions.
I don't know exactly which plugin suite you're using, but if you have any other that handles damage calculation classes and popup display, it's possible we'll have conflicts and other similar behaviors.
I added a file (Kadajah_DamagePopup(comp).zip)
It's a version with a debug mode and a parameter to try to prevent NaN. Basically, it intercepts the standard RPG Maker calculation if the plugin or variable calculation returns NaN.
Could you download and test this version?
If the error continues to occur, could you please copy the console content (F8) to offer some clues?
Thank you again for all the support, I will do my best to solve this issue.