500 Internal Server Error

[#1]

Sorry  forgot to say that Opera uses the same framework and browser engine for IE, so I doubt it will work with Opera.
Firefox is so far the best I've ever used can you please try it.

[#2]

Firefox is my normal browser and it as been for a long time. Hence the reason when the problem first occurred I tested with IE as this was already installed. Once the problem showed in both browsers. Tested both browsers via my server PC. When that failed got the wife to try from her PC. Just had a thought. never tested from my Linux PC so I will do that next.
I am open to suggestions what other browsers to try as I am using Firefox on Linux as well.
[Edit]
Just finished trying Firefox under Linux and still got the same problem. Installed Epiphany and unfortunately still getting the 500 error :(.

[#3]

The error is a server error not a browser error so its the server thats the problem.
http://pcsupport.about.com/od/findbyerrormessage/a/500servererror.htm
you could try
Solution: (in IE)
Tools >> Internet Options >> connections tab >> LAN settings >> uncheck the auto detect feature
You likely need to restart the browser, I did a full reboot to be certain.
What is the server is IIS or linux there should be something in the logs but this is a catch all error.
Is it only on your accounts it does it? If so can you try another account as it could be a permissions problem with your account

[#4]

since we are working it out slowly be elimination, we've knocked  down 3 reasons so far:
1) Browser
2) cookies
3) and another OS (you've tried Linux)
By trying the Proxy you were accessing the forum using another IP, so the forum was seeing the proxy server's IP but not your static IP. http headers are not forwarded, and no multi-piping. But if your IP was the problem why would it return 500 Internal Server Error 
do you wanna try changing your static IP to dynamic?
Has the person responsible for maintaining the forum able to get any logs or any similar situations?
To be honest, it is more like a DB issue with the forum or a problem updating some files or records. espcially that when you post, the forum needs to log this and updates the post counter, index it and logs your IP address... etc.
things like these become problematic when the script fails to write to a file or access a DB records.
If you can get someone to check the logs, they should look like:
[Date-Time] [error] [client IP] error description goes here

Lets try to find out if the dynamic IP will resolve the problem before jumping to these things

[#5]

Thanks for all the suggestions. The forum owner as set up a special test page for me. On running this test found that 1000 char submitted ok, but 10000 chars didn't. Every thing considered I think it would be worth trying the dynamic IP for a short while so I have just actioned the request.
[Edit]
Now on a Dynamic IP and with the IP change encountered exactly the same problem when editing posts and when running the test script. I am baffled. Thought the IP was the key....  :(. So I need to look at this more when I get home from work.

[#6]

Yesterday I got  a surprise, I could once again make long posts and edit posts. I thought the website owner must have been doing some tweaks in the background, but no he hadn't. So the reason I was getting the 500 error is still a mystery as is why it fixed itself. I am just glad it is sorted.