Before publishing I backuped the old config file to a temp folder within my solution by mistake. DON'T, it bites ;-) and result in compiler error:

It is an error to use a section registered as allowDefinition='MachineToApplication' beyond application level. This error can be caused by a virtual directory not being configured as an application in IIS.

If you dont know it, it can keep u bussy for a while. Error points to main config file, so doesnt give a clue.