Marco Mascheroni suggested that maybe our credmon fails to authenticate with collector, which IIUC would explain why we can't find it.
It would be your credd, but ...
/var/log/condor/CredLog in crab-sched-901 has following lines
... it looks like you looked in the right file anyway.
IDTokens in the machine's /etc/condor/tokens.d is OK since schedd talks finely with collector.
When the schedd advertises to the collector, the collector checks ALLOW_ADVERTISE_SCHEDD. (And also DENY_ADVERTISE_SCHEDD.) The credd requires ALLOW_DAEMON.
-- ToddM