The issue
After upgrading to ESG 5.x and importing all configurations from the old ESG 4.9, quarantine reports and releases requests is some cases are not sent.
Why
ESG before 4.9 allowed to upload certificates with incomplete certificate chains, which are invalid for almost any application but web clients. Starting with ESG 5.0, TLS validation is strict for every service, so the application refuses to talk to the MTA due to the (now) invalid certificate installed.
Note: this usually happens when using wildcard certificates.
Solution
Retrieve your certificate, eventually by logging into you Certification Authority panel, and download the certificate again. Once you have downloaded the certificate, upload it again to ESG from the TLS Certificates page.