>Don't know. Often 'Invalid class string' happens if a DLL is not registered but since your other (working) class is in the same assembly it doesn't seem to be the case.
>Do you get any more info is you try to start the service manually ?

Viv,

This is how that service is supposed to be executed

C:\Program Files (x86)\Siriusware\Pool\poolservice.exe Middleware

and I do start it manually, but it doesn't really start.

I'll try re-starting my laptop and see if it helps.

If not, then I'll try getting the previous version of the project file and I'll try to re-build the DLL again using the older version.