For some reason, the Remote Server which runs the Remote Metadata Adapter, after correctly connecting with the Proxy Adapter, seems either not to receive the commands, or not to respond.
Unfortunately, the Remote Server does not log its read-write activity, so a sniffing is needed in order to discriminate the two cases.

I tried to replicate the problem, but the two (piped and networked) adapter pairs work in my installation. May you please also show your adapters.xml configuration files for the two adapter pairs (i.e. NMSTEST and NMSTEST2) and the command lines for the two Remote Servers (i.e. for the Metadata Adapter and Data Adapter of NMSTEST)?

By the way, launching the Remote Server for the Remote Metadata Adapter but configuring it with the class for the Remote Data Adapter would reproduce the exception.