Unhappy [Q] HTC One M8 - ChompSMS - Message Not Sent

Have been using ChompSMS free version for over 6 months now. So far, it has been working fine, until a few days ago. It simple stopped sending SMS with an error message - message not sent - and a red exclamation mark. I can receive SMS, but not send them at this point. The deafault sms messenger is working fine, though.

Any ideas?
Thanks in advance