That’s still too many lines for me to manually check…
However, I found the reason for the problem in my case:
I had some acknowledgement comments that contained fancy unicode emojis and apparently that broke in the step of decoding the livestatus response.
I fixed it by looking through grep comment_data ~/var/nagios/retention.dat
and manually fixing the broken lines there.
I’ve reported that to the developers (FEED-7279).