What's this ??

[#1]

Your session timed out while posting. Please try to re-submit your message.
No subject was filled in.
The message body was left empty ?
Exactly how long are you allowed before this happens ?

[#2]

That's odd I've never had it happen and I sometimes leave the screen open for some time before posting.

[#3]

You have a tick box option to stay logged on, have you (accidentally maybe) changed this setting perhaps?
I don't use that option so rather expect to be told <and am periodically> that I need to re-log in. Not sure but I think its somewhere between half and one hour.
I simply copy/paste what I've typed already into the new page, copying before re-logging on of course. Then you don't have to start all over again.

[#4]

Edit/  having just had a quick look, you have the option of how long to stay logged in. Mines currently set at 60 minutes.

[#5]

i've had it happen before, for no apparent reason..

[#6]

Thanks for the responses, it has happened a couple of times before although I have changed nothing in my settings.
Seems it will remain a mystery until someone finds an explanation.

[#7]

I wonder if it related to this ? - Plusnet Forum "Connection reset by peer" error
I will keep an eye on whether my proxy server can catch an error message while posting a reply to a forum topic.