Oh noes, wrong link in previous post. The instructions link should be: http://world.episerver.com/Documentation/Items/Installation-Instructions/EPiServer-CMS/Version-7/Installation-Instructions---EPiServer-7-CMS/ 

Hi,

I did feel the pain, it took me a whole long day to find out that we should not run CMS6 and 7 side by side on production server. You will get alot of troubles (e.g: some modules wont work as it should, you will not access to cms-backend with 404 error... ) if you do that even in dev, test environments it works well. 

Did you have correct assemblybindings in web.config for all environments? Dev, test and production environments should not be that different :)

I asked this matter from Episerver support. In short, they replied that they do not know of any problems but have not fully tested the compatibility.

Yes, the configuration files in all environments are identical. I have the site works properly on dev, daily, test environments but when deploy it to staging and production lots of headaches came.  I found that even in dev environments if you have cms6 and 7 side by side sometimes you will get the described issues but rarely, and iisreset solved the problem but it wont work out on staging and production and I dont know why :).