This is a known issue (bug) with USG 4.4.28.* firmware confirmed on official beta forum, the error would be like:

Sep 26 22:38:55 main-router mcad:  ace_reporter.process_inform_response(): Failed to get the decrypted data from custom alert response#012
Sep 26 22:38:55 main-router mcad:  ace_reporter.reporter_fail(): Decrypt Error (http://192.168.1.10:8080/inform)
Sep 26 22:39:10 main-router mcad:  mcagent_data.data_decrypt(): header too small. size=0, should be=40
Sep 26 22:39:10 main-router mcad:  ace_reporter.process_inform_response(): Failed to get the decrypted data from custom alert response#012
Sep 26 22:39:10 main-router mcad:  ace_reporter.reporter_fail(): Decrypt Error (http://192.168.1.10:8080/inform)
Sep 26 22:54:31 main-router mcad:  mcagent_data.data_decrypt(): header too small. size=0, should be=40
Sep 26 22:54:31 main-router mcad:  ace_reporter.process_inform_response(): Failed to get the decrypted data from custom alert response#012

Solutions (try them in the following order):

• Disable period speed test
• Reboot the controller
• Force provision the USG
• Try official unifi-util patch
• Reset USG and readopt it
• Upgrade/downgrade to a previously working version

Mine got everything back to work after resetting the USG.

Some tips:

You can run info to get some basic info what’s going on with your USG:

user@main-router:~$ info

Model:       UniFi-Gateway-4
Version:     4.4.29.5124212
MAC Address: 78:8a:20:7c:ba:1d
IP Address:  11.22.33.44
Hostname:    main-router
Uptime:      192 seconds

Status:      Connected (http://unifi:8080/inform)