Thanks, it looks like a bug in the E1750. The E1750 hangs when Diafaan SMS Server requests the list of received messages. You can try setting the 'Modemreboot' command in the advanced settings to AT+CFUN=1,1 (this is the default value in Diafaan SMS Server 1.7.1.0) but this is not the ideal solution.
There is a newer version of the firmware available for the Huawei E1750 on the Huawei download page (search for E1750). The Huawei firmware usually only works for devices that are not branded to a specific network.
Be careful with applying new firmware, it may disable the device!
Henk