Error on registration in translated form

When registering through our french form there comes an error since a few days. On the german page it is working. It’s the same form but only some fields are in french with WPML string translation.
The exact error message is “An error occurred while processing your request!” and in the source code it has the class wpmem_msg which might come from WP-Members plugin.
Any hints what could be?

The page I need help with: [log in to see the link]