So I analysed /var/log/syslog and found what happened. The following log entry was written into the log file at a rate of SEVERAL HUNDREDS OF THOUSANDS OF TIMES A SECOND.
kernel: [3314.xxxxxx] usb 4-4: users: process 3751 (indi_qhy_ccd) did not claim interface 0 before use
grep -c "indi_qhy_ccd"