Biztalk Error: The system cannot find the file specified (Exception from HRESULT: 0x80070002)

Each time I tried to test my new mapping using BizTalk I got this error: The system cannot find the file specified (Exception from HRESULT: 0x80070002).

So the full error in the event viewer looked like this:

A message received by adapter "FILE" on receive location "RECEIVE ORDERS PCE FILE" with URI "C:\B2B.TRADCOM.COM\ORDERS\INBOUND\PCE\*.xml" is suspended.
Error details: The system cannot find the file specified. (Exception from HRESULT: 0x80070002)
MessageId: {F83F1E4B-3B09-4F9B-A9D5-F6C1BD5C2698}
InstanceID: {E162E5AB-E621-4EA1-B643-FD843D7F8D44}


I was able to map the message using Visual Studio, so I was quite sure my BizTalk mapping was correct.

Now it turned out that I forgot to sign the assembly. So the system was unable to place the assembly in the GAC. Because I used a deploy script I didn't notice the resulting warning.