Solution to Error Code: 0x8004d00e (BizTalk application installation)

It's been a while since I've written a post due to holidays and busy days..
Well, here's a new post about a problem I ran into when installing / configuring a BizTalk application.

I was installing an application in a DTAP environment and came across the following error message when I tried to start the application (in Acceptation):

xlang/s engine event log entry: NullReferenceException exception occurred while the XLANG/s runtime enlisted a service.Error message:Object reference not set to an instance of an object.

And:

Unable to communicate with MessageBox BizTalkMsgBoxDb on SQL Instance xxx. Error Code: 0x8004d00e. Possible reasons include:  1) The MessageBox is unavailable. 2) The network link from this machine to the MessageBox is down. 3) The DTC Configuration on either this local machine or the machine hosting this MessageBox is incorrect.

After trying several blogs and forums, also with Error Code: 0x8004d00e I didn't find anything useful.

Then I saw two different versions of each dll in the GAC (MSIL and x86).

When I checked my DEV, I noticed the platform was set to "Any CPU", which had to be x86, because the A & P environments are running in 32 bit. After making a new MSI and importing it, I didn't have any issue anymore with starting the application. Another lesson learned!