Hi; many thanks. I just emailed the logs to that address.
The problem has come and gone several times. Occurred about 1.5 months ago, lasted for ~1.5 weeks, then went away again. It has come back again for the past week. We have updated the server and client a couple of times during this period. The client is definitely on the 9.1.24; the server is either on that, or the previous version. However the problem seems to persist across versions.