Can't save snapshots to disk before running in window

I am unable to save snapshots to disk if I have only been running my session in fullscreen mode-- it doesn't ask me for a filename or location, or give me any error message, just does the camera sound and animation and then quietly fails. If I switch to windowed mode, I can save snapshots normally, and I can continue to do so for the rest of the session even if I switch back to fullscreen.

I am running client version 1.18.5(3) under Ubuntu 7.10, with an nVidia GeForce2 MX 400 with 32Mb and the proprietary nVidia driver version 1.0-9631.

...and it seems this doesn't apply to postcards-- I can send them successfully regardless whether I have been in window mode. (I can't test uploading because I have no L$.)