There is an update to this issue at the foot of the piece.
I have been disatisfied with Octopi's default timelapse videos, and installed octolapse today, to try to improve things. The timelapses shown on the videos of Octolapse look very much better.
Since then, sadly, I have only been able to print one item, which did not record a timelapse.
All other prints have failed because of a "printer not primed" error.
In the octolapse menu it says that Octolapse is enabled, but doesn't mention it running. I see in some of the videos I've watched that it says both.
I am using prusa slicer 2.3.1 on a Prusa i3 Mk3 and with a default Pi camera.
The print which did work was printed in vase mode, with stabilisation at back right, Trigger at Smart-snap to print, Rendering at MP4 60fps, Camera at Default OctoPi 0.16.0 and logging of all errors. I am printing with a 0.6 nozzle, and have set my layer height to 0.46 in both the slices and Octolapse, if that helps. I have tried printing not in vase mode, but normally, and get the same error. The print is cancelled every time, until I disable octolapse.
I have set Octolapse so that it doesn't cancel the print on startup errors, but that has made no difference.
In the helpfile it tells me that the answer is to " Open your printer profile settings and find the Priming Height within the Layer Change Detection setting" .
Unfortuately, having looked for 30 minutes, I cannot find anything vaguely like this in any of the menus.
Any advice on how to print with this addon would be gratefully received.
In the meantime I have prints to do, so shall sadly return to the dreadful default timelapses, and see if I can find a way to the menus mentioned above between prints. (4 days later, and I still haven't found anything about priming in the menus! - Aargh!)
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Update
I have found the priming bit - you need to set up a custom profile in order to get to it. I had not seen that key fact.
I have set priming height to 0.5 (just greater than my default layer height), and will see what effect that has on my next print.