Sorry, it’s not really clear yet what you’ve done. Did you run the sample I pointed to? Is it also throwing an exception?
When I run the sample, it works properly, with no exceptions thrown, so I would assume that would be the same for you.
Are you saying that in your code it is the DisableTimeoutManager call that throws the exception? Or are you saying that removing that call in your code stops the exception?
In your code, are you still referencing other NServiceBus packages? Are you creating multiple endpoints?
The goal at this point is to try to figure out what is different about your code compared to the sample.
Getting back to the version discussion:
To answer your question about why not using the latest versions, I’m using these specific version because those are the versions that I’ll need to implement the solution after the POC.
NServiceBus 6.4.3 is no longer supported. The only version of NServiceBus 6 that is still supported is 6.5.10.
NServiceBus 6 itself is about to go out of mainstream support. Support ends for it on 2020-05-29. After that you need to have an extended support contract. See Support Policy • NServiceBus • Particular Docs for more details.
I highly recommend moving to the latest/supported versions as soon as possible.