Trying to Post New Thread goes to 'Page Not Found'


When I try to create a new thread I get sent to 'Page Not Found'. A couple things happen here:

1. If it's a pretty short post it will post.

2. If it's longer it goes to 'Page Not Found' but the error page is from my blog (my forum is in a subdirectory and my blog is in the root).

My webhost made some changes to the .htaccess and I thought everything was fixed but apparently not.

Any help on how to troubleshoot this would be great.

I wanted to update:

If I post a thread with a small amount of text (100 words or less approx) it will post, and then I can keep adding to the post using the 'edit' link as much as I want.

But if I post a larger amount at once, it will throw the error I outlined above.
What is the address in the address bar when you get the page not found error?

What add-ons do you have installed?
It's the same as when I'm posting the thread:

Auto Follow on Registration 1.0

AWeber Integration for XenForo 1.0

Digital Point Ad Positioning 1.1.0

Postloop Integration 1.0.2

[bd] Widget Framework 1.5.4

I don't think it's an addon because this started when my server admin screwed around with tht .htaccess
Here you go Chris:

#    Mod_security can interfere with uploading of content such as attachments. If you#    cannot attach files, remove the "#" from the lines below.#<IfModule mod_security.c>#    SecFilterEngine Off#    SecFilterScanPOST Off#</IfModule>ErrorDocument 401 defaultErrorDocument 403 defaultErrorDocument 404 defaultErrorDocument 500 default
<IfModule mod_rewrite.c>   
RewriteEngine On   
#    If you are having problems with the rewrite rules, remove the "#" from the    #    line that begins "RewriteBase" below. You will also have to change the path    #    of the rewrite to reflect the path to your XenForo installation.   
RewriteBase /hangar   
#    This line may be needed to enable WebDAV editing with PHP as a CGI.   
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]   
RewriteCond %{REQUEST_FILENAME} -f [OR]   
RewriteCond %{REQUEST_FILENAME} -l [OR]   
RewriteCond %{REQUEST_FILENAME} -d   
RewriteRule ^.*$ - [NC,L]   
RewriteRule ^(data/|js/|styles/|install/|favicon\.ico|crossdomain\.xml|robots\.txt) - [NC,L]   
RewriteRule ^.*$ index.php [NC,L]