Edit: After rebuilding my presets one by one, it seems that the cause was a preset switch action I had triggering on "After Launching On Mac With Serial Number". It seems that BTT relaunches itself as soon as a screensaver starts, and this action caused the screensaver to quit.
I'm resolving this by changing the trigger for my preset switches to named actions, and using an Applescript in my User's Login Items to trigger these on startup for each of my Macs.
I'm seeing this too. The screensaver starts but exists within a couple of seconds. With BTT closed, this doesn't happen. But with BTT running, this happens 100% of the time even after a restart.
If I look closely at the menu bar, it looks like BTT is being relaunched when it comes out of the screensaver - the BTT icon disappears momentarily and then reappears in the menu bar.
How can I check if BTT is running a script? I have a Switch to Preset run on startup, but otherwise there shouldn't be anything running at the time the screensaver starts.
Edit: if I disable the attached preset, the problem disappears. But I'm having trouble identifying which of the actions is causing the screensaver to exit.
Edit: if I force quit BTTRelaunch in activity monitor before the screensaver starts, then the problem doesn't occur. Obviously this is not a fix, but maybe sheds some light on the issue?