Problema de ensamblaje firmado

There is a WCF service in my project. It's referencing another assembly with service contract interface. Everything worked well until I signed reference with contract using key and made changes into the contract and service.

After rebuild i started getting such exception: Method XXX in type YYY from assembly ZZZ does not have an implementation.

Seems that service is using the old dll with a contract but can't find the reason why. When i look through the generated dlls using reflector - there is no old code - signature of contract and service are identical. If i remove signing from the contract - everything works perfect.

In test purposes service is working under Cassini web server. Also tried to run it under IIS. My previous solution - was to delete ASP.NET temporary files. Now it doesn't help.

The reference with contract is signed for sharepoint needs. The error also disappears if i remove this reference from GAC. But why service project is looking for this dll in GAC? Is there a workaround?

Agradecería cualquier sugerencia.