[Resolved] New registration is not possible

Hi! I recently relaunched my website to a new WP theme and since then the registration of a new affiliate is not possbile anymore. I tested to fill out the registration form in 2 szenarios:

1) The box Automatically approve a new affiliate is checked:
–> After filling out the registration form a critical error appears: The Design of my Website is crashed and it says: There was a critical error on your website. Learn more about WordPress troubleshooting.

2) The box Automatically approve a new affiliate is not checked:
–> After filling out the registration form, I want to approve the new Affiliate but then an error message appears: ERROR: Attempting to load invalid message from ‘aff_approved_email_existing_user_account’. There was a critical error on your website.

I tried it with several email adresses (one of them I created new to be 100 % sure, it wasn’t used before) but every time I get the same error.

What can I do? I allready updated the plugin to the latest version (1.0.1).