Error after updating SCG to 5.18.00.20. An alert file failed to transfer from gateway.

Ever since we upgraded to SCG 5.18.00.20 we've started to regularly receive alerts: "An alert file failed to transfer from gateway. Something went wrong while sending an alert file from gateway with the hostname/IP..." It keeps saying "/var/log/connectemc/logs/ConnectEMC_MFT.log" failed. Rebooting doesn't help.

The file contents are:

[2023-09-21 09:26:10] MFT session details...
[2023-09-21 09:26:10] File[1]=/opt/connectemc/poll/RSC_CF2ZV213100147_092123_015143332.cec. Size=5352
[2023-09-21 09:26:10] Successfully decrypted key

[2023-09-21 09:26:10] MFT transfer timeout: 900 seconds
[2023-09-21 09:26:10] MFT transfer IP protocol family: IPv4
[2023-09-21 09:26:10] mft_ptr->mft->url:
[2023-09-21 09:26:10] https://localhost:9443
[2023-09-21 09:26:10] MFT transfer timeout: 900 seconds
[2023-09-21 09:26:10] USE_HOSTNAME_AUTO - CURL_IPRESOLVE_V4
[2023-09-21 09:26:10] https://localhost:9443/esrs/v1/devices/SECURECONNECTGATEWAY-APPLIANCE/SERIALNUMBERREMOVED/mft
[2023-09-21 09:26:10] https://localhost:9443/esrs/v1/devices/SECURECONNECTGATEWAY-APPLIANCE/SERIALNUMBERREMOVED/mft/RSC_CF2ZV213100147_092123_015143332.cec
[2023-09-21 09:26:10] Signature generated successfully
[2023-09-21 09:26:10] Auth string = SECURECONNECTGATEWAY-APPLIANCE:SERIALNUMBERREMOVED:0Kx+B8Uals2cxmrtl7YZ13WT5zg=,domain=Device
[2023-09-21 09:26:10] Starting the CURL transfer ...
[2023-09-21 09:26:10] CURL Transfer complete
[2023-09-21 09:26:10] mft failed The requested URL returned error: 502 Proxy Error, error: 22, cmd: https://localhost:9443/esrs/v1/devices/SECURECONNECTGATEWAY-APPLIANCE/SERIALNUMBERREMOVED/mft
  Trying http://127.0.0.1
TCP_NODELAY set
Connected to localhost (http://127.0.0.1
ALPN, offering http/1.1
TLSv1.2 (OUT), TLS handshake, Client hello (1):
TLSv1.2 (IN), TLS handshake, Server hello (2):
TLSv1.2 (IN), TLS handshake, Certificate (11):
TLSv1.2 (IN), TLS handshake, Server key exchange (12):
TLSv1.2 (IN), TLS handshake, Server finished (14):
TLSv1.2 (OUT), TLS handshake, Client key exchange (16):
TLSv1.2 (OUT), TLS change cipher, Client hello (1):
TLSv1.2 (OUT), TLS handshake, Finished (20):
TLSv1.2 (IN), TLS handshake, Finished (20):
SSL connection using TLSv1.2 / ECDHE-RSA-AES256-GCM-SHA384
ALPN, server accepted to use http/1.1
Server certificate:
 subject: C=US; ST=Texas; L=Round Rock; O=Dell Technologies Inc.; OU=Dell Secure Remote Services; CN=http://scg.example.com
 start date: Feb  2 17:17:35 2023 GMT
 expire date: Feb  2 17:17:35 2043 GMT
 issuer: C=US; ST=Texas; L=Round Rock; O=Dell Technologies Inc.; OU=Dell Secure Remote Services; CN=http://scg.example.com
 SSL certificate verify result: unable to get local issuer certificate (20), continuing anyway.
PUT /esrs/v1/devices/SECURECONNECTGATEWAY-APPLIANCE/SERIALNUMBERREMOVED/mft/RSC_CF2ZV213100147_092123_015143332.cec HTTP/1.1
Host: localhost:9443
Accept: */*
ProductFamily: SECURECONNECTGATEWAY-APPLIANCE
SerialNumber: SERIALNUMBERREMOVED
User-Agent: CONNECTEMC5.18.00-1
TransferType: ConnectHome
CHAuditProductFamily: ISILON-GW
CHAuditSN: REMOVED
Date: Thu Sep 21 09:26:10 2023
Authorization: SECURECONNECTGATEWAY-APPLIANCE:SERIALNUMBERREMOVED:0Kx+B8Uals2cxmrtl7YZ13WT5zg=,domain=Device
Content-Length: 5352
Expect: 100-continue

HTTP/1.1 100 Continue
We are completely uploaded and fine
The requested URL returned error: 502 Proxy Error
stopped the pause stream!
Closing connection 0
TLSv1.2 (OUT), TLS alert, Client hello (1):
[2023-09-21 09:26:10] MFT Transfer ended.