Getting a fork started is certainly a start 🙂
I haven't really had a look at or even used previous versions of HumHub so I imagine it taking some time to find out exactly what changed with the newer version that broke old themes. My first guess though from the internal error and "It looks like you may have taken a wrong turn" messages in addition to one of the pull requests on github that suggests changing "$canWrite" to "$canEdit" almost makes me think there was a bunch of changes to how data is referenced that causes a mismatch in what the theme is attempting to reference because it's reference point is completely different. Could be wrong though; I'm not terribly experienced in web development.
