Hi @kitelab
TL;DR:
The premium version of our plugin works with WPML & Polylang, where you can set an extra condition to only show field groups per language (see screenshot here). This way, you can simply duplicate your field groups and translate them right within the plugin.
More info:
I had a look and the problem seems to be with qTranslate-X (and qTranslate-XT has the same problem) where they crash if you don’t use __() when using their shortcode [:en].
A solution is to not use [:en] and translate the options in another way. I understand the plugin offers an easy way of doing things for you, but it’s not really a WordPress standard so I can imagine not many plugins support this syntax.
Furthermore, the plugin hasn’t been updated in 3.5 years and according to the comments, many people are having issues. I would strongly suggest you look at changing to an updated language plugin. I understand qTranslate-X has some features you like, but if it’s unmaintained, it’s only going to give you (security) problems down the road. Your site should use up-to-date plugins so it benefits from speed and is more secured against hackers :-).