How to reproduce:

1. Open a new org file and paste this outline:

* x
* a
** a 
* b

2. Place your cursor at the headline b
3. Trigger evilem-motion-find-char-backward and enter the a key. The command will run without a problem.
4. Now fold the top heading a and return to b.
5. Repeat step 3. This should freeze up Emacs, forcing you to use C-g to stop the command
6. Trigger evilem-motion-find-char-backward again, this time search for x. Emacs won't freeze in this case.
7. Now place your cursor at the headline x
8. Trigger evilem-motion-find-char-forward and search for 'a'. This command will run successfully too.

So it seems that evilem have trouble searching a character backwards, but only if that character is within a folded subtree.