Unable to cast COM object of type 'System.__ComObject' to interface type

I am really pulling my hair out at this one. I have literally no idea why I am having these issues.

Right, I have 3 SSIS packages. One for development, one for user acceptance testing and one for a live release. Each package is pretty much an exact copy of the development one, just using different connection strings.

The development and UAT package work perfectly fine, no issues. And have been working for the 6 months since they've been created. I came to creating the LIVE package two days ago. I made a copy of the UAT package and changed connection strings. All was well. I then closed the package and it began validating the connections. It began taking a long time to validate the connections compared to the other two, in fact. It seems to repeatedly validate the same connections over and over again. Eventually, after about 10 minutes it stops and gives me this error message:

Unable to cast COM object of type 'System.__ComObject' to interface type 'Microsoft.SqlServer.Dts.Pipeline.Wrapper.IDTSObject100'. This operation failed because the QueryInterface call on the COM component for the interface with IID '{D4E5AF42-7999-473C-8082-6EFC676953C4}' failed due to the following error: The application called an interface that was marshalled for a different thread. (Exception from HRESULT: 0x8001010E (RPC_E_WRONG_THREAD)).

I looked this up online, I have followed the instructions on registering my DLLs. No changes at all. I uninstalled the MariaDB (MySQL) instance from ther server. Reinstalled it, amended all the settings and gave it another crack. This time I posted a previous question on how to fix the problem. Someone told me that I cannot create a copy from the other packages unless I change the GUID. I didn't believe this to be true as I had done so before. Anyway, I copied the package and changed the GUID. This again, did not work. So instead I created a whole new package and created the control flow again and again. This did not work.

I am at a loose end and have no idea why this just will not work. Our application is slated for release in less than 2 weeks and this issue just will not leave. Can anyone help?

EDIT: THE QUESTION YESTERDAY DID NOT RECEIVE A CORRECT ANSWER. Please do not close this question.