no further info, just
2017-03-10 20:20:20: (connections-glue.c.291) connection closed - read failed: Function not implemented 38
2017-03-10 20:20:20: (connections-glue.c.291) connection closed - read failed: Function not implemented 38
2017-03-10 20:20:20: (connections-glue.c.291) connection closed - read failed: Function not implemented 38
2017-03-10 20:20:20: (connections-glue.c.291) connection closed - read failed: Function not implemented 38
2017-03-10 20:20:20: (connections-glue.c.291) connection closed - read failed: Function not implemented 38
2017-03-10 20:20:20: (connections-glue.c.291) connection closed - read failed: Function not implemented 38
2017-03-10 20:20:20: (connections-glue.c.291) connection closed - read failed: Function not implemented 38
2017-03-10 20:20:20: (connections-glue.c.291) connection closed - read failed: Function not implemented 38
2017-03-10 20:20:20: (connections-glue.c.291) connection closed - read failed: Function not implemented 38
2017-03-10 20:20:20: (connections-glue.c.291) connection closed - read failed: Function not implemented 38
2017-03-10 20:20:23: (connections-glue.c.291) connection closed - read failed: Function not implemented 38
2017-03-10 20:20:53: (connections-glue.c.291) connection closed - read failed: Function not implemented 38
2017-03-10 20:20:54: (connections-glue.c.291) connection closed - read failed: Function not implemented 38
2017-03-10 20:20:56: (connections-glue.c.291) connection closed - read failed: Function not implemented 38
2017-03-10 20:20:56: (connections-glue.c.291) connection closed - read failed: Function not implemented 38
2017-03-10 20:20:56: (connections-glue.c.291) connection closed - read failed: Function not implemented 38
2017-03-10 20:20:56: (connections-glue.c.291) connection closed - read failed: Function not implemented 38
2017-03-10 20:20:58: (connections-glue.c.291) connection closed - read failed: Function not implemented 38
2017-03-10 20:20:59: (connections-glue.c.291) connection closed - read failed: Function not implemented 38
2017-03-10 20:21:05: (server.c.1828) server stopped by UID = 0 PID = 28707
2017-03-10 20:21:06: (log.c.217) server started
2017-03-10 20:21:06: (server.c.1295) WARNING: unknown config-key: setenv.set-environment (ignored)
2017-03-10 20:21:40: (connections-glue.c.291) connection closed - read failed: Function not implemented 38
2017-03-10 20:21:43: (connections-glue.c.291) connection closed - read failed: Function not implemented 38
2017-03-10 20:22:03: (connections-glue.c.291) connection closed - read failed: Function not implemented 38
2017-03-10 20:22:10: (server.c.1828) server stopped by UID = 0 PID = 29736
2017-03-10 20:22:11: (log.c.217) server started
2017-03-10 20:22:11: (server.c.1295) WARNING: unknown config-key: setenv.set-environment (ignored)
I am thinking if this because new set env module, or lighttpd as itself.
Edit: And I was right :-]