Here is my theory on that because it happens a lot and I think it has to do with how long I spend on an entry. At some point, it seems like the system "times out." Here is what I do, one or the other:
1. Prepare in a text editor, then cut and paste into the
gallery entry so my time spent is a lot less on the actual
gallery entry page
2. Before clicking "save," I copy the text in case this happens. If it does, I do a new entry with the cut and pasted text.
In both cases, this always seems to
work for me. I can spend some time on the
gallery text page because I always have to rearrange things to fit the formatting I use for
gallery entries.
Hope this helps.
Virgil