Error "Unable to gather" - mime: no media type

Hi, I’m running influxdb 2.0.6 and am seeing an error in /var/log/syslog every 10 seconds:

May 5 12:25:46 aserver influxd[38194]: ts=2021-05-05T16:25:46.344845Z lvl=error msg="Unable to gather" log_id=0TwK~Tzl000 error="mime: no media type"

Every second I have a process POSTing to /api/v2/write to add new data using the line protocol with the Content-Type header set to ‘text/plain’.

I’ve turned on debug logging and can see there are no other requests coming in, so am wondering how to figure out what is causing the error.
Any ideas?