Looks like there’s something odd - an unicode error - in one of the log files you are scanning. I am guessing that you haven’t made a change to the pattern files… can you confirm this?
You may be able to isolate which file by settting up a test pattern and running blacklist test.
See nftfw/How_do_I.md at master · pcollinson/nftfw · GitHub.
I’ll see what can be done to make this more resiliant.