This happens (more often as of today) when I try to post something. Usually I can click the back button and repost it, as it is convenient enough to remember what you had written, but it's still an inconvenience.
This is an error with the website host. Upon further research, it appears other people hosting phpBB3 forums with our same host have had this problem as well. Unfortunately, we can't switch hosts because our current host is free and part of the forum switch was to stop paying for a website.
I've had this issue too, though it hasn't happened recently.
Alright. Luckily, as I said before, you can just hit the back button and try again. At least THAT'S something quite convenient with this forum. Enjin, as far as I know, didn't have that feature. lol