MyBB Community Forums

if i create short message on new thread its work fine
but error screen if i create a long message on new thread 

Forbidden
You don't have permission to access /newthread.php on this server.
Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request.

The 403 Forbidden error is an HTTP status code that means that accessing the page or resource you were trying to reach is absolutely forbidden for some reason.

The 403 error is essentially saying "Go away and don't come back here."

How to Fix the 403 Forbidden Error

1. Check for URL errors and make sure you're specifying an actual web page file name and extension, not just a directory. Most websites are configured to disallow directory browsing so a 403 Forbidden message when trying to display a folder instead of a specific page is normal and expected.
   
   NOTE: This is, by far, the most common reason for a website to return the 403 Forbidden error. Be sure you fully explore this possibility before investing time in the troubleshooting below.
   
   
2. Clear your browser's cache. Issues with a cached version of the page you're viewing could be causing 403 Forbidden issues.
   
3. Log in to the website, assuming it's possible and appropriate to do so. A 403 Forbidden message could mean that you need additional access before you can view the page.
   
   Typically, a website produces a 401 Unauthorized error when special permission is required but sometimes a 403 Forbidden is used instead.
   
4. Clear your browser's cookies, especially if you typically log in to this website and logging in again (the last step) didn't work.
   
5. Contact your ISP if you are still getting the 403 error, especially if you're pretty sure that the website in question is working for others right now.
   
   It's possible that your public IP address, or your entire Internet Service Provider, has been blacklisted, a situation that could produce a 403 Forbidden error, usually on all pages on one or more sites.
   

Also try contacting your webhost.

contact your web host and ask if they have added additional security like mod_security / Suhosin.
give them information about above referred problem and request to fix the issue by changing security rules.

solve my problem with submit open ticket in my webhosting