It can also happen if the server is busy.
Heres a tip:
BEFORE you post, SELECT ALL and COPY your message
Then Post
If it fails, you can start again, and PASTE your message into a new posting.
Also, try clicking the BACK button,
It happened this morning with me, and the BACK button worked. It took the Submit second time around.
Finally, if your posting a "War and Peace" article, its one thats most likely to fail due to timeout (and the one that is most frustrating, to put it mildly).
In that case, write your article in a text editior (e.g. WORD) and copy and paste it into a Forum mesage.