I can see my dongle as HSPA modem and create a new connection in the network configuration gui of KDE. But when I try to connect, it says 'preparing to connect' but can not connect.

I've installed gnome-logs and seen that there are some error messages regarding missing 'ppp' plugin for network manager. I've installed Networkmanager-ppp package. Now those errors are gone, but some other errors are coming ...

cdc_wdm 2-1.3:3.5: wdm_int_callback - 0 bytes cdc_acm 2-1.3:3.3: failed to set dtr/rts cdc_acm 2-1.3:3.4: urb 5 failed submission with -19

Can some one help me how to debug this ?