Recently I tried to get into customizer of wptouch and it returns error 500. I deactivated all the plugins and reactivated them one by one and I found that the error occurs when both GP premium and wptouch are enabled. I think that it is a conflict between these two plugins.
I also contacted the plugin author of WPTouch, and here’s the reply:
Sorry to hear about the trouble you’re having. This usually happens when another one of the active plugins or the desktop theme is loading its scripts in WPtouch Pro’s Customizer. WordPress advises against doing this, as it may cause issues like what you’re seeing.
Thank you for letting us know about this. We’ll note the conflict in our Knowledge Base.