OK, bug happened again - got the log.
I figured out what happened with the 'disappearing' log section. I selected the log with 'browse' but didn't post until after I tried to reproduce the bug - and then, of course, the new log had overwritten the file I wanted to attach.
Switching between bases was a red herring - this happened on the first base I clicked on from the geoscape when I hit the 'Airplane' button.
The base did have one plane (Stilleto) out at the time, that might be relevant.
Note that the content of the log differs slightly (from memory) compared to last time, but it's basically the same problem of looping commands overrunning string buffer.
[EDIT] P.S. That was with a nightly-build I downloaded after my first post.