Operating system name and version: Ubuntu 12.04 LTS
Version of smsd: 3.1.14
Smsd installed from: package repository
Name and model of a modem / phone: Huawei E3276
Interface: USB

Modem fails to register with the mobile network. smsd_trouble.log shows:
2013-12-05 11:13:50,5, smsd: Outgoing file checker has started. PID: 1635. 2013-12-05 11:13:50,5, smsd: Everything ok now. 2013-12-05 11:13:52,7, GSM1: -> AT+CREG? 2013-12-05 11:13:52,7, GSM1: Command is sent, waiting for the answer 2013-12-05 11:13:53,7, GSM1: <- +CREG: 0,2 OK 2013-12-05 11:13:53,5, GSM1: MODEM IS NOT REGISTERED, WAITING 1 SEC. BEFORE RETRYING 1. TIME 2013-12-05 11:13:54,7, GSM1: -> AT+CSQ 2013-12-05 11:13:54,7, GSM1: Command is sent, waiting for the answer 2013-12-05 11:13:54,7, GSM1: <- +CSQ: 10,99 OK 2013-12-05 11:13:54,5, GSM1: Signal Strength Indicator: (10,99) -93 dBm (Workable), Bit Error Rate: not known or not detectable 2013-12-05 11:13:54,7, GSM1: -> AT+CREG? 2013-12-05 11:13:54,7, GSM1: Command is sent, waiting for the answer 2013-12-05 11:13:55,7, GSM1: <- +CREG: 0,2 OK 2013-12-05 11:13:55,5, GSM1: MODEM IS NOT REGISTERED, WAITING 1 SEC. BEFORE RETRYING 2. TIME ...

And so on for 100 times, after which smsd_status shows blocked. When testing with another Ubuntu 12.04 server a day earlier with the same conf, everything worked fine.
Where should I look for the cause?

Issue resolved itself after multiple modem unplug/plug "operations".