Going into the /drv/log and removing the files for future recordings would have done the trick. That is just what Latty did, only with a shell command:
ok... so basically I went to /home/hts/.hts/tvheadend/dvr/log and did a
rm `grep -L filename\"\: *`
After a restart, tvheadend nicely went though the EPG database and applied my autorec rules. My failed recordings list is empty again and the upcoming recordings are there. I probably could have improved the grep to only find future events, it also removed the past failures...
https://tvheadend.org/issues/2356