Recreated by placing a loom and using white_banner/[any dye]/globe_banner_pattern.  Placing the three latter items into the loom causes the client to disconnect from the server with the following exception:

 [20:22:50] [Server thread/INFO]: thenameisKO lost connection: Internal Exception: io.netty.handler.codec.EncoderException: java.lang.NullPointerException

Attempted to recreate on Vanilla 1.14 without success, banner creation acts normally in single-player vanilla.  This was tested in Spigot 1.14 without any plugins installed.  The exception is thrown as soon as the last item is placed into the loom's crafting grid; the player is unable to remove the banner before crashing.  Once reloaded, the player will be standing in front of the loom and the crafting ingredients will be on the ground.  No errors are output in the client's log.  This was reported to me by another server administrator and I was able to recreate it continuously.  This was able to be recreated/verified in less than 15 minutes (assuming one can readily access an updated Spigot 1.14 server without any plugins installed).