The same problem if I run the Freeswitch "testserver" application of ESL library: the "event plain" command receives always a reply "-ERR command not found" and also the "linger" command.
[INFO] testserver.c:19 mycallback() Connected! 4
[DEBUG] src/esl.c:1141 esl_send() SEND
filter unique-id 9e159038-d599-4e18-afe9-a8f4eeffd76b
[DEBUG] src/esl.c:989 esl_recv_event() RECV HEADER [Content-Type] = [command/reply]
[DEBUG] src/esl.c:989 esl_recv_event() RECV HEADER [Reply-Text] = [+OK filter added. [unique-id]=[9e159038-d599-4e18-afe9-a8f4eeffd76b]]
[DEBUG] src/esl.c:1141 esl_send() SEND
event plain SESSION_HEARTBEAT CHANNEL_ANSWER CHANNEL_ORIGINATE CHANNEL_PROGRESS CHANNEL_HANGUP CHANNEL_BRIDGE CHANNEL_UNBRIDGE CHANNEL_OUTGOING CHANNEL_EXECUTE CHANNEL_EXECUTE_COMPLETE DTMF CUSTOM conference::maintenance
[DEBUG] src/esl.c:989 esl_recv_event() RECV HEADER [Content-Type] = [command/reply]
[DEBUG] src/esl.c:989 esl_recv_event() RECV HEADER [Reply-Text] = [-ERR command not found]
[DEBUG] src/esl.c:1141 esl_send() SEND
linger
[DEBUG] src/esl.c:989 esl_recv_event() RECV HEADER [Content-Type] = [command/reply]
[DEBUG] src/esl.c:989 esl_recv_event() RECV HEADER [Reply-Text] = [-ERR command not found]
[DEBUG] src/esl.c:1141 esl_send() SEND
sendmsg
call-command: execute
execute-app-name: answer
[DEBUG] src/esl.c:989 esl_recv_event() RECV HEADER [Content-Type] = [command/reply]
[DEBUG] src/esl.c:989 esl_recv_event() RECV HEADER [Reply-Text] = [+OK]
[DEBUG] src/esl.c:989 esl_recv_event() RECV HEADER [Content-Type] = [text/disconnect-notice]
[DEBUG] src/esl.c:989 esl_recv_event() RECV HEADER [Controlled-Session-UUID] = [9e159038-d599-4e18-afe9-a8f4eeffd76b]
[DEBUG] src/esl.c:989 esl_recv_event() RECV HEADER [Content-Disposition] = [disconnect]
[DEBUG] src/esl.c:989 esl_recv_event() RECV HEADER [Content-Length] = [67]
[INFO] testserver.c:49 mycallback() Disconnected! 4