This is the exact same problem as #3743.
When a message writeEnd or readBegin are called the context is not cleaned leading to possible undefined state of the context.
The error was visible for me when i would make 2 RPC calls with the same protocol instance, producing invalid messages like [<message1>],[<message2>]. The comma is a syntax error and invalid JSON and protocol.
Resetting the context as is done for java fixes the issue.