Multicam mode is one of the bugs currently in VPX13! It is being "worked on" by the developers. At the moment, we users must just wait patiently for the fix, there is no workaround!
If you have a multicam project that needs to be finished urgently, this post, from a member of Magix staff, explains exactly how you can successfully revert to VPX12.
UPDATE: I see a new patch has just been released which says that "multicam mode is fixed"!