Error while spliting messages

[bharat]

When I try to split messages and click on the other page I get this error:

403 - Forbidden Error
You are not allowed to access this address.
If the error persists, please contact the website webmaster.
If you are the webmaster of this site please log in to Cpanel and check the Error Logs. You will find the exact reason for this error there.

Common reasons for this error are:

Incorrect file/directory permissions: Below 644.
In order files to be read by the webserver, their permissions have to be equal or above 644. You can update file permissions with a FTP client or through cPanel's File Manager.

Restrictive Apache directives inside .htaccess file.
There are two Apache directives which can cause this error - 'Deny from' and 'Options -Indexes'.

What can be the reason?

[SychO]

Check with your host if mod_security is enabled, ask them to disable it

[bharat]

Where can I check it?

[bharat]

Somehow I get this message only when I try to split messages in one particular topic.

[SychO]

Where can I check it?

Contact your host

[bharat]

I can use the same option on all other topics except one and it is really strange.

[Kindred]

See above statements.  Contact your host