troubleshooting advices are welcome, openbsc website was no help
The clock of the non working one might just be too off ...
See http://openbsc.osmocom.org/trac/wiki/isdnsync for a tool to calibrate (with appropriate HW).
If you don't have the equipement ... huh, good luck. You can always try random values until it works. If you have a USRP you can try to lock onto it and compare offset ...
Cheers,
Sylvain