Further experimentation has narrowed the problem down to the Alpha yoke.
It seems to be related to switch positions but not as simple as one being on or off. For example, with the ALT half of the master switch on and all other switches off, I can consistantly log in. Adding a second switch to on can bring the problem back but adding a third can make it work again.
I won’t bother trying all the combinations as there is no point. Just glad to have a work-around for the problem.