You are having problems accessing the Control Center when TLS is disabled on any supported Internet browser.
You simply cannot access the Control Center anymore because "Internet Explorer cannot display the web page".
Due to FIPS 140 compliance some changes were implemented in the Control Center to make sure it is always using TLSv1/SSL.
The Control Center will always operate on FIPS mode even if it is disabled to provide better security.
All current web browsers are TLS 1.0 enabled and support FIPS approved cipher suites.
You must have TLS 1.0 enabled in the web browser to access the Control Center on versions 9.5.2 and later. It is not possible to achieve the same result by enabling TLS 1.1 or TLS 1.2 only.
Please also take following information into account when upgrading to SMG 10.5.2:
Unable to view the Control Center for Symantec Messaging Gateway (SMG) in Internet Explorer after update to SMG 10.5.2
Symantec Messaging Gateway 9.5.2 or later