Hi Croft--I had this exact same issue with my SS HC freezing and displaying strange symbols when trying to change the motor rate. Everything up to pressing those keys seems to work normally and the mount (AVX) is still controllable using CWPI even with the HC in the frozen state.
I reported the problem to Celestron a couple of weeks ago and they responded that they were working on a SS HC firmware update that should fix this issue. Today I received a follow up email stating that the new HC firmware (1.22.22307) had been released and could be installed via CFM.
I haven't had a chance to try it yet, but will try it and report back.
Regards,
Jeff