This addon either needs retiring or taken over by another developer, as clearly the current dev has no time to fix its bugs. Its not the first time either.

Its not combining most videos, and hasn't been for months. 1 out of 4 videos might combine, but with it downloading videos as slow as it does, who the hell will try 4 times for a successfull combine.

Other competitor addons exist, and they work on youtube + many more video sites.