We have an extension to ArcMap 10.6.1 that retrieves image files from Micro Focus' Content Manager via its .NET SDK. If and only if we establish a connection to the Content Manager (CM) Workgroup Server, New Document "crashes" the ArcMap GUI leaving an ArcMap process running in the background that needs to be killed using Task Manager. I have cut down our code so that it does nothing at all with the CM connection other than immediately close it down again, dispose the "Database" connection object and even force a garbage collection cycle. No matter, the fact that I have made the connection at all is enough to break the New Document functionality. It is almost as though the problem is just the presence in memory of the CM SDK dll.
Watching the ArcMap table of contents, it looks like the current document is closed OK and that the "crash" occurs while the new one is being created. No errors are reported. No exceptions are thrown. We just lose the GUI without complaint or explanation.
This all worked fine with ArcMap 10.2.1 and the corresponding predecessor version of CM.
Can anyone suggest what the problem might be or how I might investigate it?