First thing to do to avoid loosing your post content is to preview it first because in doing so, you already have it saved on the draft, then you may proceed to hitting the post button, if you get the same error response, then try to close the browser and reload the page and the post content from the draft, it will send, if you continue to see this repeated, i will advise that you either check your browser settings or you make use of another browser.
You meant about forum's draft page.
If you have problem with your log in session, your Internet connection, Preview and Draft page does not work and if you don't have proper action, you will lose the content.
To avoid this, a safer approach is to compose your post on a third party tool like Microsoft Word, Note pad before pasting it to the forum, click on Preview, click on Post. If anything bad happen, you still have the content in your word composing tool.