For some reason I realized NXlog wasn't sending logs to graylog (after previously doing so flawlessly)

I went to the Nxlog log and found this just before it stopped responding:

What does this mean? I can't find anything online, but I know the udp arguments work because restarting nxlog works fine.

2020-03-09 08:29:36 ERROR om_udp apr_socket_send failed; An invalid argument was supplied.

However, Graylog received a message 20ms later from that machine (the last message that was sent before nxlog went offline)

Any ideas?