Gateway connection failing to SQLServer

We have a dataflow that uses dynamic datasource connecting to various servers in a loop through an on-prem gateway. This has worked fine for over 2 years. But after a recent upgrade from SQLServer 2014 to 2019 (RTM-CU19) (KB5023049) - 15.0.4298.1 (X64), we get this error when refreshing the dataflow for some of the tables/queries on the dataflow (different ones for each refresh):

Error Code: Mashup Exception Error, Error Details: Couldn't refresh the entity because of an issue with the mashup document MashupException.Error: Microsoft SQL: A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: TCP Provider, error: 0 - An operation on a socket could not be performed because the system lacked sufficient buffer space or because a queue was full.) Details: DataSourceKind = SQL;DataSourcePath = tcp:10.66.3.53,3814;HRMTS.Customer.XXXX;Message = A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: TCP Provider, error: 0 - An operation on a socket could not be performed because the system lacked sufficient buffer space or because a queue was full.);ErrorCode = -2146232060;Number = 10055;Class = 20 GatewayObjectId: cfe029ff-3c00-4fe3-a086-c07aa0a3df5f (Request ID: c46e9f0f-4712-4b98-b3e0-5d74060166ab).

We are looking into the SQLServer configuration too, but is there anything I can change in the attached gateway config file to mitigate this?

Link to the gateway config file being used:

Gateway config 

As I mentioned, this config file worked fine when we used SQLServer 2014.