The main problem is caused by your search form. In ASP.NET WebForms there can be one and only one form element on a page and the page already has one but you've added another one inside it and I believe the closing form element of your added form is being interpreted as the end of the main form by ASP.NET.
The way to solve that is to implement your search as a user control and embed the user control in the master page. Then in your user control insterad of using a form element use an asp:Button and set the postback url to the url of your search page and that will make it post to that page using the existing form on the page.
Other than that it seems your skin won't work with CombineCss set to true, I can probably fix that for you and send it back to you but I would possibly need to re-organize a few things in your skin folder. Even without combining css I had to copy the font and image folders out of the css sub folder into the main skin folder to get it to work locally.
A few other things I would do:
- Remove IncludeHtml5Script="true" from the <portal:IEStyleIncludes since you are already using modernizer
- Remove greybox and oomph from style.config since they are no longer needed or used
- reset.css should probably be at the top of style.config
- I'm not sure where you are adding the csspie but I would put it in the root and include using ie cmments since its only needed in ie
Let me know if you'd like me to try and solve the css combining thing, its a lot of css filles and with them not combined it adds a lot of http requests which can slow down the page load time. Also without combining them you can't really control the caching because the browser will cache those static urls.
Hope that helps,