Just a note to let you know I solved the problem. It turns out I had failed to move over to v3 my simconnect.xml file. Once I did this all worked fine.
This is not normal. The simconnect.xml file is not needed, if you just run P3D and some addons on the same machine.
The only time you need to worry about the simconnect .xml file, is if you need to setup a client-server connection, with P3D as a server running on one system, and some client addons running on another one.
Maybe you had also a Simconnect.cfg (which is also NOT needed when running the sim on a local install) that you HAVE moved to V3 ? That
would explain the problem: if you have a Simconnect.cfg (used by clients), you must also have a matching Simconnect.xml (which is the server), even if they are both the same machine. However, if they are both the same machine, BOTH files are NOT needed and they should BOTH removed.
Unless, you really have a need for a networked client-server configuration (if you use a 3rd party P3D client that connects over a network), and in that case, you need BOTH files correctly configured.