MOVEit Transfer site server not accessible after Windows OS upgrade from 2012 R2 to 2019 on the Windows server hosting the Automation application OR some other Windows patch was installed on the Windows server hosting the Transfer (DMZ) application. The Test connection from the DMZ host in Automation is failing with the following message: 

Error testing host : 2850 Could not list folders: could not access URL' https://<address>:443/machine.aspx: HttpSendRequestEX failed: The attempt to connect to the server failed'

The error could also be as follows:
Could not list folders: Could not access URL: HttpSendRequestEx failed: Error establishing SSL connection.

There also should be Schannel errors in the MOVEit Automation server windows event viewer with the following message: (An TLS 1.0 connection request was received from a remote client application, but none of the cipher suites supported by the client application are supported by the server. The TLS connection request has failed).

SSL ciphers required for the secure connection between the Automation server and the Transfer server are disabled on the Windows server hosting the Automation application and this happened after the OS upgrade.

Another possible cause could be that in the Transfer config, under the path tab, the base URL is setup incorrectly and the moveitdmz site binding for port 443 has a hostname in it.

Another possible cause can be filtering products. If a filtering solution on the Automation end is blocking the connection, that can cause this error message. This can affect connections to MOVEit Cloud as well, so if an admin does not have access to the Transfer server, check to ensure the Automation server does not have filtering solutions blocking. 

Another possible cause is the presence of an internal Firewall between MOVEit Automation and Transfer  where an access policy is interfering with the MOVEit Automation connection to MOVEit Transfer 

If the above steps do not resolve the issue, try the following:

Refresh the DMZ host settings in automation i.e.  re-enter the account name and password, uncheck the option 'ignore cert errors'  save the settings then go back and check the 'ignore cert errors' button again   then  test the connection again to see if that works.

Additionally, check to see if there are any filtering solutions present. In previous cases, an organization encountered this error in Automation connection to a Transfer host (MOVEit Cloud organization) due to a Palo Alto Networks URL Filtering Solution. 

As mentioned in the possible cause, an internal firewall could have an access policy interfering with the connection between MOVEit Automation and Transfer ; so please look into this possibility if the above workarounds do not help