If you encounter this error message in Hitfilm Pro and Hitfilm 4 Express:
GL_INVALID_OPERATION – The specified operation is not allowed in the current state.
The offending command is ignored and has no other side effect than to set the error flag.
Then you have to check the effect which you have been applied with the video one by one. To diagnose, tweak each of effect in your clips, I would say try to remove some effects if you have no idea which single effect cause trouble. In my case it was a Text effect which used Keyframe cause this break down. How I found it? I released that whenever I remove this Keyframe attribute the Viewer doesn’t goes slow anymore, just runs as normal behavior.
Hope this help a little. Cheers!