I get the same thing after upgrading to iOS 8.0.2
For variety I also sometimes get:
The operation couldn't be completed. (NSURLErrorDomain error -1012.)
I found if I do a hard reset (on/off button and home button held down together) then I can login the FIRST time only. Does this 'work' for you?
nope!
We had to have Collector completely killed, not just running in the background, for the hard reset to work.
I received this error in the middle of working on ArcGIS Online on my desktop PC - asking for my username and resetting the password allowed me to log back in again.