Inbound/Outbound calls fail, VoIP Gateway responds with 502 Bad Gateway

[sbphone]

Our previously stable 3CX has been almost completely disabled. No configuration changes had been made, although the problems started soon after a brief internet outage. All incoming calls receive a busy signal and then disconnect, and all outbound calls get a Server Internal Error. Event logs for outbound calls look like this:

• Call or Registration to 4034675943@(Ln.10000@Patton 4114) has failed. 192.168.1.48 replied: 502 Bad Gateway; from IP:192.168.1.48:5066
  SIP Server ID: 12294 04/25/2022 9:42:32 PM
• Call or Registration to 4034675943@(Ln.10000@Patton 4114) has failed. 192.168.1.48 replied: 502 Bad Gateway; from IP:192.168.1.48:5064
  SIP Server ID: 12294 04/25/2022 9:42:32 PM
• Call or Registration to 4034675943@(Ln.10000@Patton 4114) has failed. 192.168.1.48 replied: 502 Bad Gateway; from IP:192.168.1.48:5062
  SIP Server ID: 12294 04/25/2022 9:42:32 PM
• Call or Registration to 4034675943@(Ln.10000@Patton 4114) has failed. 192.168.1.48 replied: 502 Bad Gateway; from IP:192.168.1.48:5060
  SIP Server ID: 12294 04/25/2022 9:42:31 PM

This is using a Patton 4114 VoIP gateway, as you may have noticed above. The Patton appears to be working and can be accessed via both Telnet and web interface, although the VoIP Link light just blinks, indicating a lack of connection/registration. I can ping the Patton from the 3CX server, and vice versa.

System information:

3CX: Standard Annual 16.0.8.16
Server OS: Windows 10 Home 21H1
Type: Locally hosted
Phone: Yealink SIP-T38G 38.70.0.228
Provisioning: Local (?)
VOIP Gateway: Patton 4114
Firewall checker passed

Thank you so much for any assistance!