Hello!
Skype and IP are ntot a problem, though I'd like to avoid holding a conversation on skype if possible (we can chat through here alright for most cases).
Now on teh matter:
1) This happens on every one account at once or just the one being edited?
2) When it does happen reloading the page/navigating to a know URI in the admin gets the same 'blank screen of death' result or it loads alright if teh URI is typed in directly?
3) Do you have profile/membership management theme/plugins installed and active there?
4) Have you done any recent changes to server(configuration) or wp prior to this started happening (if you can recall)?
5) The sandbox server installation is 'exactly teh same' as the production one and it's happening there as well?
Regards,
Dobri