I logged a ticket with WPML that has been escalated to their compatibility team. They advise me to mention this to you, and ask for further support.
At WPML i called the ticket “Invalid login language switching”, and it has been treated by one Shekhar Bhandari.
The issue : on http://149.210.164.31/heinpoblome/
go to another language than english;
go to the ‘my login’ page (‘mon profil’ in french or ‘mijn login’ in dutch);
login with a non-existant user;
the page will switch to the ENGLISH invalid login page;
This is when WPML diagnosed this as a compatibility issue. I will also log this ticket with Formidable, since there does not seem to be a clear ‘culprit’.
Can you help?