ceph commands fail with the message 'librados: client.admin authentication error', why?

Issue

• Ceph commands such as ceph -s (and others) fail with the message "librados: client.admin authentication error".

• The errors can be as following:

2017-12-11 22:53:08.597962 7fbfcc59f700  0 -- 192.168.1.1:0/1031610 >> 192.168.1.3:6789/0 pipe(0x7fbfbc000bb0 sd=4 :0 s=1 pgs=0 cs=0 l=1 c=0x7fbfbc017090).fault
2017-12-11 22:53:11.598663 7fbfcc6a0700  0 -- 192.168.1.1:0/1031610 >> 192.168.1.4:6789/0 pipe(0x7fbfbc0113f0 sd=4 :0 s=1 pgs=0 cs=0 l=1 c=0x7fbfbc004fa0).fault
2017-12-11 22:53:17.600665 7fbfcc7a1700  0 -- 192.168.1.1:0/1031610 >> 192.168.1.2:6789/0 pipe(0x7fbfbc0113f0 sd=3 :0 s=1 pgs=0 cs=0 l=1 c=0x7fbfbc0182d0).fault
2017-12-11 22:53:23.601871 7fbfcc59f700  0 -- 192.168.1.1:0/1031610 >> 192.168.1.4:6789/0 pipe(0x7fbfbc0113f0 sd=3 :0 s=1 pgs=0 cs=0 l=1 c=0x7fbfbc017a80).fault
2017-12-11 22:53:32.603520 7fbfcc7a1700  0 -- 192.168.1.1:0/1031610 >> 192.168.1.4:6789/0 pipe(0x7fbfbc000bb0 sd=4 :0 s=1 pgs=0 cs=0 l=1 c=0x7fbfbc00b560).fault
2017-12-11 22:53:38.604612 7fbfcc6a0700  0 -- 192.168.1.1:0/1031610 >> 192.168.1.4:6789/0 pipe(0x7fbfbc000bb0 sd=4 :0 s=1 pgs=0 cs=0 l=1 c=0x7fbfbc009250).fault
2017-12-11 22:53:44.606349 7fbfcc59f700  0 -- 192.168.1.1:0/1031610 >> 192.168.1.2:6789/0 pipe(0x7fbfbc000bb0 sd=3 :0 s=1 pgs=0 cs=0 l=1 c=0x7fbfbc00e500).fault
2017-12-11 22:53:47.549533 7fbfce717700  0 monclient(hunting): authenticate timed out after 300
2017-12-11 22:53:47.549663 7fbfce717700  0 librados: client.admin authentication error (110) Connection timed out
Error connecting to cluster: TimedOut

• This leads to the client not able to connect to the cluster.