I upped the limit a while back from 60,000 characters per post to 100,000 characters - so unless your post exceeds 100,000 characters, there shouldn't be an issue.
What your describing usually occurs when there's a character in the body of the text that phpBB doesn't like.
Try copying your text into notepad, and then back into a NEW post - this is especially relevant if the body of your text has been created in a Microsoft program. See if that works.
If what you're trying to do with your current post is create a new topic, then try just adding it to an existing topic in the Test forum.
Failing this, feel free to send me a copy of what you're trying to post to email@example.com and I'll take a look.