Last tested: Feb 4, 2019
One of the reasons when users all of the sudden cannot login into Looker using their SSO SAML credentials, could be as simple as running out of space on the machine housing Looker app.
what that could look like in logs:
[WARN|461eb2|cache] :: Error writing cache entry for 744927ea4758e179f895938bda89bde1ab3a23ac. No space left on device - /home/looker/looker/.cache/744927ea4758e179f895938bda89bde1ab3a23ac.ruby_bin.kSCbqKr2.tmp
The solution would be to either free up some space or mount a bigger drive.