I've recently come across a problem with the bat form: it seems players are able to activate bat form day or night, and either way it results in reduced health, but not flight and not physical transformation.

Error that appears in server console

Player screenshot showing health change but no other effects

I do have a rather ridiculous number of mods but I was hoping you might be able to help me nonetheless

Did not mean to close it yet.

I will upload a new alpha later today or tomorrow, so you can test if the problem is fixed

Sorry, I hoped I would be able to release a new alpha with some new features by today, but it's not finished.
So please try this build (it is based on your current version, but contains a potential fix for your issue): https://maxanier.de/maven2/de/teamlapen/vampirism/Vampirism/1.10.2-1.1.0-alpha+01-16-14-51/Vampirism-1.10.2-1.1.0-alpha+01-16-14-51.jar

Did you try the build already?

Sorry for the delay. I can confirm that bat mode now works perfectly with this version

Perfect :)