GM862-GPS not connecting to network anymore

My GM862-GPS module worked all right in the beginning, but now it does not connect to the GSM network anymore. The GPS part still works. After switching the module on, the Status LED starts flashing fast and after a little while it flashes slow as if connected to the network. I already checked the SIM card in another phone without any problems.

This is the output on the serial port.

at+cpin?                                                                                        
+CPIN: READY                                                                                    
                                                                                                
OK                                                                                              
                                                                                                                    
at+creg?                                                                                                                    
+CREG: 0,4                                                                                                                  
                                                                                                                            
OK 
                                                                                                                         
at+csq                                                                                                                      
+CSQ: 14,0                                                                                                                  
                                                                                                                            
OK  

at+cops                                                                                                                     
ERROR

Is there a way to force the module to connect to the network?

:idea: