Can't start the listener

I'm having a weird problem that I haven't run into before. The listener is refusing to start with the errors below. Sysadmin says there's no firewall on the server. Any ideas what could cause this? Oracle db version is 12.2.0.1.0.

TNS-12547: TNS:lost contact

TNS-12560: TNS:protocol adapter error

TNS-00517: Lost contact

Linux Error: 104: Connection reset by peer

Contents of listener.ora file:

# listener.ora Network Configuration File: /u01/app/oracle/product/12.2.0/db1/network/admin/listener.ora

# Generated by Oracle configuration tools.

LISTENER =

 (DESCRIPTION_LIST =

   (DESCRIPTION =

     (ADDRESS = (PROTOCOL = TCP)(HOST = hostname.com)(PORT = 1521))

     (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1521))

   )

 )

Last listener.log entry: