additionally, a 500 internal server error error was encountered while trying to use an errordocument to handle the request.
Many times, this is caused by an incompatibility issue between one/multiple plugin/s that you use or the theme you are using.
Please try this:
- disable all plugins except WPML and its add-ons - check if the issue is happening again - if it's not, reactivate your plugins one by one, checking every time if the issue shows up again so that you can understand which plugin is causing that issue
If the issue happens even with the non-WPML plugins disabled:
- leave all the non-WPML plugins deactivated - switch to a default theme like TwentySeventeen - try to recreate the issue now