Thanks Kyle, forgot to mention I had done that too. No errors. Activity already upgraded to latest on problem site.
They're similar sites, same server provider.
The site where it works properly has an older version of cbactivity, I will upgrade that now as I'm writing and see if makes a difference: still works.
I dunno, everything seems to be eliminated, system actions can't be changed without knowing it, same versions, there's no on/off switch. Stuck with it maybe?
Joomla 3.6.2
CB 2.0.15 Latest Nightly
GJ 3.0.1 nightly
Php version 5.5.35
Auto actions nightly
Activity nightly
Privacy nightly
Conditions nightly
CB Gallery 2.0
.....all nightly!
Uddeim 3.9