Enable limits had been left on in EQMOD (by mistake), and on the last image of a sequence, the mount hit this limit.
I think SGP was trying to recover, as the plate solving window was open when I first looked.
SGP was very sluggish to refresh its display, and eventually would not respond to any mouse input.
Task manager showed that SGP was using about 5-8% CPU time. In the end I had to kill the task.
(Sorry don’t know how to include the logfile).