The 3.5.3 beta addressed that problem. I had a good imaging session last night. Thanks!
tldr; I think I see a new instability in the most recent Ekos version, possibly tied to the dynamic image stacking feature; Plausible? Known? Next steps?
I've been using KStars/Ekos/indilib via astroberry on an RPi 4 for the past year or so, to good effect. While it's been a steep learning curve, I've always been able to identify *my* problem thanks to the documentation or this forum or occasionally blind luck stumbling onto the right setting to change and learning a new term at the same time, at least until now.
Within the past couple of weeks, I was motivated to upgrade to a newer version (to support the Canon Ra, but I think that's a side issue). I started with the newest stable image (2.0.3), and applied the recommended upgrade commands: "sudo apt update && sudo apt upgrade && sudo apt dist-upgrade". Once I did that, I started seeing a crash in Ekos, or occasionally all of KStars after taking 10 images or so in the photograph sequence tab. There didn't seem to be any such trouble with e.g. the plate solving tab. Not seeing any missing patches and also not having support for the Canon Ra in that most recent astroberry version, I picked up stellarmate; Good news! The Canon Ra is supported! Bad news! It's crashing in the same way after 10 images or so (the number isn't entirely consistent, but it's small).
Given that I'm seeing the same behavior in both the astroberry distro and stellarmate OS, I'm inclined to point my finger at one of the recently introduced features. In each session, as I'm watching the images in the preview pane being stacked, I can see some artifacts in the image (almost like frost on a lens -- made me check my lenses each time -- no frost) that become more pronounced until the crash. Purely circumstantial evidence.
I have not collected logs yet -- I'll do that on my next session (Friday night, if the weather holds), now that I know how.
As to versions; I've done the update as of today, so the versions are the current stable ones for astroberry, and the stellarmate OS install is equally current, no updates available as of this post.
Thanks in advance.