Resolution:
If you see the exact following error in repo debug log:

15:48:33 00003dc8 t000042b8 iclib: config DEBUG=F:\tibco\products\bwworkflow\5.1\server\IST_LONDON_ICSERVER\logs\repd_debug.log
15:48:33 00003dc8 t000042b8 iclib: config SERVER-PATH=F:\tibco\products\bwworkflow\5.1\server\IST_LONDON_ICSERVER\bin\icrepserver.exe
15:48:33 00003dc8 t000042b8 iclib: config PORT=9001
15:48:33 00003dc8 t000042b8 iclib: config MODE=Authenticated
15:48:33 00003dc8 t000042b8 iclib: config LOG-FILE=F:\tibco\products\bwworkflow\5.1\server\IST_LONDON_ICSERVER\logs\repserver.log
15:48:33 00003dc8 t000042b8 iclib: config LOG-LEVEL=debug
15:48:33 00003dc8 t000042b8 iclib: config VALID-DIR=F:\tibco\products\bwworkflow\5.1\server\RepositoryServer_IST_LONDON_ICSERVER
15:48:33 00003dc8 t000042b8 iclib: config VALID-HOST=all
16:32:42 00003dc8 t000042b8 iclib: Starting new authenticated server
16:32:42 00003dc8 t000042b8 iclib: all hosts are valid
16:32:42 00003dc8 t000042b8 iclib: received response TCP_GETKEY
16:32:42 00003dc8 t000042b8 iclib: now0=1140019200, now1=1140015104, now2=1140023296
16:32:42 00003dc8 t000042b8 iclib: rk1=777855FF rk2=39C3FFD5
16:32:42 00003dc8 t000042b8 iclib: received response TCP_AUTH
16:32:42 00003dc8 t000042b8 iclib: calling _IC.IST_LONDON_ICSERVER.DISP.3.110005
16:32:42 00003dc8 t000042b8 iclib: calling _IC.IST_LONDON_ICSERVER.DISP.3.110005
16:32:42 00003dc8 t000042b8 iclib: calling _IC.IST_LONDON_ICSERVER.DISP.3.110005
16:32:42 00003dc8 t000042b8 iclib: calling _IC.IST_LONDON_ICSERVER.DISP.3.110005
16:32:42 00003dc8 t000042b8 iclib: IcClient::validateToken failed (Failed.)
16:32:42 00003dc8 t000042b8 iclib: received response TCP_AUTH
16:32:42 00003dc8 t000042b8 iclib: calling _IC.IST_LONDON_ICSERVER.DISP.3.110005
16:32:42 00003dc8 t000042b8 iclib: calling _IC.IST_LONDON_ICSERVER.DISP.3.110005
16:32:42 00003dc8 t000042b8 iclib: calling _IC.IST_LONDON_ICSERVER.DISP.3.110005
16:32:42 00003dc8 t000042b8 iclib: calling _IC.IST_LONDON_ICSERVER.DISP.3.110005
16:32:42 00003dc8 t000042b8 iclib: IcClient::validateToken failed (Failed.)
16:32:42 00003dc8 t000042b8 iclib: Authentication failed

Please check the reposit.id file to see if you have a new line after the id.
If you do NOT have a new line, please add one and restart Workflow Document Repository server.
This should fix this problem.

Workflow Document Repository server fails with "Authentication Failed" error.What could be the problem?