Broken Links Reported that are not broken. Please help!

[orders20]

I am getting 404 broken link errors after I crawl my site. Site has over 15K pages but only 185 crawled.  Does anyone know what the problem is?  URL below...

[external links are visible to admins only]

thanks,
Dan

[XML-Sitemaps Support]

Hello Dan,

your site software returns 404 http code for the most internal pages. Although they are displayed correctly in the browser, they will never be indexed by search engines. You should contact your site developer to correct this.