Avoiding Doom - can't login to multi-domain subsite admin

WP 3.5.1 Multisite Subdomain Network
VPS php5.3.x
Multi-domains
Domain-Mapping

I just had to do a full restoration because of a 500 server error getting thrown by whatever issue is messing up settings for Multi-domains/domain mapping and adding subsites.

I've been unable to login to multi-domain subsite admin (I've read 10 threads in various places about this but never found definitive guide on how to resolve) -- entering correct user credentials just reloads the page, incorrect throws message about cookies.

this issue escalated when I tried to delete domain mapping for one of my multidomains and kaboom 500 server error and need for full restoration

I want to have ProSites allowing customers to add subsites to multi-domain sites. should be easy, right...

mainsite.com
client1.mainsite.com
client2.mainsite.com
secondsite.mainsite.com
client3.secondsite.com
client4.secondsite.com

SuperUser can't login to client2.secondsite.com, but it displays live page

deactivated ALL plugins except Domain Mapping, no
multi-domains with Single Signon - on and off, no
cleared cache and cookies, no
changed browser, no
domain mapping login mapping, original, no
domain mapping login mapping,domain entered by user, no
domain mapping login mapping, original domain, no
dashboard access through Parent dashboard, no
created new user with Editor status, no can't login
created new user with Administrator status, no can't login

please advise how to diagnose/resolve