TL;DR: UNINSTALL ALL THE MODS THAT CHANGE HOW YOUR HEALTHBAR LOOKS. OR ONES THAT SEEM LIKE THEY DON'T, BUT FOR GOD KNOWS WHAT REASON THEY DO. LOOK, IT'S COMPLICATED.

Long version and explanation:

CHAPI stands for Custom Health API. It is a library that was initially created for Fiend Folio to support any kind of custom health one could wish for. This library is also embeddable into any mod that might want to use it, one of which is Repentance Plus.
CHAPI is now fully released, polished and open-sourced. Go thank FF devs (especially TaigaTreant) for the amazing work they did and ask them any questions directly related to it, if you have any!

Unfortunately, CHAPI requires a sole access to the HP graphic files located in the game. This means that a mod that uses CHAPI can not co-exist with any mod that attempts visual tweaks of player's healthbar (some of the mods don't change anything at all, they just have unnecessary files in their folders that cannot be overridden by CHAPI). Here's the list of mods that conflict with CHAPI (it may not be complete, so if you found any other mod that causes the issue, let me know):

• Curse of the Thinking Emoji;
  
• Mantle Rework;
  
• Visible Holy Mantles;
  
• Visible Lost Health [REP];
  
• Nemesis;
  
• Co-op Plus.

If you have a mod conflict of that kind, there is no way to resolve it but to uninstall that mod or Repentance Plus. You can attempt to change the contents of those mods locally on your computer, but I wouldn't recommend doing that as 1) you may not understand whether these files are necessary for the mod, and 2) if that mod updates, your local deletions will be overridden from the workshop, forcing you to repeat the procedure.

Thanks for your understanding! This thread can and will only contain questions and answers, if there are any; other comments will be removed.