If we try to sign in as a member from the ACP we just get the front end login page.
It's not entirely consistent, maybe 1 out of 30 tries it works. Have not found any pattern.

You would get the login page only if you are not logged in as admin on the front end before you begin. Could that perhaps be the case?

If you are logging into multiple accounts, you must log out of that impersonated user before logging into another account. 

Meaning if Admin1 logs into Member1, before logging into Member2… you must logout (choose switch back to Admin1 from the front end).  If you don’t, my experience is you are logged out of the front end entirely. 

Many thanks, that solved the problem.