How can I diagnose a problem in the SMS module when opening a modem?

I see the following line in the log:
DEBUG: modem_process: opening modem

After about 5 minutes, I see the next one:
ERROR:modem_process: cannot open modem Siemens_S45! Resource temporarily unavailable

All happens in libsms_modem.c, line 423, where open() is called. The call blocks for about 5 minutes and afterwards I see the above error in the log.

I tried to connect to the modem via minicom (on /dev/ttyS0) and everything is just fine.

Thanks,

Cristian