Microsoft has released security updates for vulnerabilities found in:
Exchange Server 2013
Exchange Server 2016
Exchange Server 2019
These updates are available for the following specific...
Updated May 04, 2021
Version 8.0
Blog Post
Long shot, but I've seen free/busy break between servers after disabling TLS 1.0/1.1 (and thus only supporting 1.2 and higher).
Doubt that's the case here, but trying doesn't hurt I presume.
Below registry keys will configure .NET to use higher TLS levels. Apparently it doesn't default to TLS 1.2 if it's available and not even when it's the only stack enabled.
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v2.0.50727]
"SchUseStrongCrypto"=dword:00000001
"SystemDefaultTlsVersions"=dword:00000001
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v4.0.30319]
"SchUseStrongCrypto"=dword:00000001
"SystemDefaultTlsVersions"=dword:00000001
[HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\.NETFramework\v2.0.50727]
"SchUseStrongCrypto"=dword:00000001
"SystemDefaultTlsVersions"=dword:00000001
[HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\.NETFramework\v4.0.30319]
"SchUseStrongCrypto"=dword:00000001
"SystemDefaultTlsVersions"=dword:00000001