Solved after a complete reinstall of 5.2.6.1 from scratch. Back to the old, good usage.
I've noticed before that a fresh reinstall helps. Which made me look at some registry entries used by the software to see what was different.
In some cases (and this was a few years ago so I don't remember the details) there must have been some version in-between that set some configuration entry different, and then maybe future versions changed it back or something, but only on a fresh install. Meanwhile, that registry entry remained in a (now) non-default state which caused whatever.
I vaguely remember in my case it had something to do with an audio setting on particular cameras. Existing cameras couldn't play audio, but if I re-added it using the exact same camera settings, it worked fine. Comparing the two showed a slight difference.
Anyway, it could be something like that.
I'm wondering in general how reliable it is to simply re-install the update EXE for a previous version? Besides the handful of EXE files that are in the update, I'm guessing there are also some registry changes made - changes to default behavior which might not switch back when you downgrade? I would guess there's some mechanism for version control, because people wouldn't be expected to install interim updates from 5.1 all the way up to the latest 5.2.6.1, so each update would include any other non-file changes (if there are any). Maybe it really is only the files, and there's no other registry things happening? That'd be good... I just always wondered if downgrading was really the exact same thing as if you installed that older version from scratch.
I guess when in doubt, reinstall using the full installer for whatever version, which is what I've done when I got myself into a jam. it usually helps, as long as I'm going back to a known-stable version. The new update features in 5.2.6.0 should help in that regard.