Have you diagnosed the system log to determine if there has been any change since the modem was detected?
After the modem is installed, grab a copy of the system log:
dmesg > ~/modem-in-log.txt
Also check that the device is present and accessible, for instance does the modem support AT commands? Try sending AT<CR> to get a response from it.
After the problem has occurred, grab a follow-up copy of the system log via the same procedure above, but redirecting to a different file name like "modem-fail-log.txt" and see if anything is different with respect to the ACM0 device.
Re-check that the device is present and accessible, retry AT commands and see if the modem is still working.
If the modem is still working, then it may be the application or server software you're using to perform your connections. Try stopping and restarting that.