Howdy Mark,
Thanks for bearing with our slower responses over the holidays. We are back at capacity now and catching up.
That is interesting. Permalinks can be a bit fickle, because there are numerous ways in which other plugins can accidentally manipulate them. Even simply changing the order can have a bad effect. Could you try following this guide? While you should follow it in order, I think that once you get to the the Less Common Solutions > Conflict test you might find something. But the first heading might reveal an issue to. In particular, what happens if you change your permalink setting temporarily? Perhaps to anything but your current setting or Default.
Please let me know the result of those steps, or if you have any questions. Thanks!
– Brook