Error calling ctor of signed COM .Net DLL

if've got a problem with a sigend .Net COM DLL and I try to call the ctor of this class in VB6 but it only works on my machine, but not on the other developer machines. The error is 0x80131534.

The Class looks like this:

// GUIDs are examples. In my component they're diffrent
[ComVisible(true)]
[Guid("AAAAAAAA-AAAA-1111-AAAA-AAAAAAAAAAAA")]
[ClassInterface(ClassInterfaceType.None)]
[ComSourceInterfaces(typeof(IServiceEvents))]
[Transaction(TransactionOption.NotSupported)]
public class X : Service, IX
{
    public X() : base() { }
    // some code here.
}  

[ComVisible(true)]
[Guid("AAAAAAAA-AAAA-1111-AAAA-AAAAAAAAAAAA")]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public interface IX { }

[ComVisible(true)]
[Guid("AAAAAAAA-AAAA-1111-AAAA-AAAAAAAAAAAA")]
[ClassInterface(ClassInterfaceType.None)]
[Transaction(TransactionOption.NotSupported)]
public abstract class Service : System.EnterpriseServices.ServicedComponent, IService { }

[ComVisible(true)]
[Guid("AAAAAAAA-AAAA-1111-AAAA-AAAAAAAAAAAA")]
[InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
public interface IServiceEvents { }

And is registered like this:

gacutil /i MyComp.dll
regasm MyComp.dll /tlb
regsvcs /appname:MyApp MyComp.dll

The developers got the source, compieled it and the rgistry thing is correct. If I or they compare the signed keys with

sn -vf MyComp.dll

we'll get "Assembly 'MyComp.dll' is valid."

Has anyone an idea, what I do wrong?

Thanks in advance for your help!