I encountered this error (reason why I was not able to join in a private server).
At the time of the error I was using WebDisplays-0.11 and MCEF-1.7.10-0.6.
I know there is a new version actually, I'm sure gonna use that, btw, I'm not able to reproduce the issue anymore (for now...)
This is WebDisplays related and not MCEF. Besides, I'm dropping WebDisplays 0.11 support since I'm working on a re-write, so these kind of bugs should be solved.
I encountered this error (reason why I was not able to join in a private server). At the time of the error I was using WebDisplays-0.11 and MCEF-1.7.10-0.6. I know there is a new version actually, I'm sure gonna use that, btw, I'm not able to reproduce the issue anymore (for now...)