If that is the case, please provide a crash report as well as a list of other mods installed.

I'm unsure how much I can help here, but I'm having the same issue. It seems to be a crash specific to version 1.4.4, as downgrading to 1.4.3 fixed the problem for me.

I have very few mods loaded - JEI, Mystical Lib, Mystical World, Patchouli, and Roots.

I had the issue on the Forge verison of 14.23.5.2847. Here's the full crash log:
crash-2019-09-13_18.15.58-client.txt

Not sure if I can do anything to help otherwise, but I hope this sheds some light.

That's... super-strange. I'm going to check the upload of 1.4.4 to make sure it wasn't somehow corrupted. Thank you so much for the further information, this actually allows me to start investigating it!

Okay, so digging into the actual .jar that's available on CurseForge, the EntityOwl definitely exists, and the point at which this error is occurring (initializedMods) is late enough in the process that Mystical World should have already loaded... I'll dig even deeper.

So, I'm not able to replicate this. The only thing I can recommend doing at this point is deleting 1.4.3 and reinstalling 1.4.4, as potentially it was a corrupted or invalid mod file. It doesn't seem to be happening any more, but I've had a number of issues where files that I've uploaded, even though they've been stated as the correct size, are being reported as identical to an "empty file"...