Though not fully sure, however this is a very generic error which is more to do with the network…try to connect to ftp without IS and see how it goes … if you can reach the ftp server then try to validate the certificates related to the FTPES in IS see if it’s all as expected …
Yes please check the certs/connectivity on both sides…mostly its with SSL connectivity…unless it was working before and assuming this a new connectivity testing:
While connecting from your home PC, did the partner FTP server ask you to validate your credentials or was it anonymous? IF you supplied the credentials and if it worked then check certificates in IS…
also it would be wise connecting your partner system from your work network / Domain to make sure the IP and port is not blocked by your firewall…
what do you mean by trusted directory? you also need to import the certificated/ public key in your truststore file