I understand you use your own method of sending SMS text messages with some kind of email-to-SMS conversion or gateway.
Since version 18.104.22.1688 we updated, among many other things:
- The default email notification templates,
- the method of how PRTG sends emails (especially regarding encrypted connections),
- and the encoding of emails
For details, please see the Stable Release Notes and search for all appearances of "notifications".
So, I guess there is some incompatibility between the "new" email format PRTG is now using and your existing gateway.
For debugging, please try sending the email notifications to your personal email inbox where you can analyze the messages sent by PRTG. As a next step you can then spot the issue and adapt your conversion or gateway accordingly.