I have a partial resolution to my export/import problem. I eventually
discovered that we had lost IP connectivity between the Linux TSM
server and the TCP/IP address space used by the z/OS TSM server.
Once the network configuration was fixed I was able to do the
exports.

I would not expect the TSM server code to be able to do server to server
exports without network connectivity, but I would expect it to do a
decent job of reporting the cause of the failure, and I would expect
it to do a decent job of cleaning up after the failure (that is, not
leaving administrative sessions that are impossible to cancel).