Spark 2.8 is using completely new Smack library (2.7 used 2-3 years old insecure Smack library). A lot has changed in Smack in those years, so some things are broken in Spark 2.8. File transfer is very complex thing (possibilities of network/routing setups are endless). And Spark doesn’t have competent developers to fix such issues. For me 2.7 also worked better with file transfers between different networks (e.g. in my virtual environment with different subnets). 2.8 works sometimes, sometimes doesn’t. Usually it works ok in the same subnet (like a local LAN). If you MUST have working file transfers, i suggest to use 2.7.7 version.
I can only file a ticket for this, but there is nobody to work on it. [SPARK-1869] Degraded file transfer feature when sending files between different networks - IgniteRealtime JIRA