One I can help with!
Since V9 I have had this issue across many many CCUs. There's no reason for it.
The ONLY fix I found was in Security Settings > Services
In here, both Configuration and Security Administrator are set to Secure, Low
Changing this to Unsecure only has fixed all of my logging in issues.
I appreciate there may be people who say that's wrong, stupid, etc. but we have them behind other security and this is the only way I've been able to log in to Manager without fail every time.