TP-LINK modem

[pteasy]

I have a problem whit this modem.
First time works everything fine, but the next time when i start the program dump GSM error 303.

[31.7.2015 8:44:30] nrGsm: send:AT+CNMI=2,2,,1\0Dh
[31.7.2015 8:44:30] nrComm: Input data length: 19
[31.7.2015 8:44:30] nrGsm: GSM Error:  303