Strange…
Could you try upgrading to v2.9: Microscope Server 2.9 and new SD images - Announcements - OpenFlexure Forum
Then from the camera settings lower the stream quality to something really low like 25? I’ve no idea if it’ll help, but it’s worth a go.
Edit: Ignore my previous comment. Contrary to the PiCamera documentation, the MJPEG quality setting seems to do literally nothing.
I’m pushing out a 2.9.1 patch release which replaces the quality option with a bitrate option. Note though that lowering the bitrate can impact fast-autofocus performance! I’ve done some non-extensive tests and it seems like it still works well enough in most cases, but your milage may vary.
Once this is out, try loweing the stream bitrate one level at a time and see if that helps.