Over some upgrades (it's been so many years I can't remember what version I originally bought), the button/selection to assemble incompletes has gone missing. I still use Quickpar and your autopar isn't up to the job.
Problem: Downloaded two files that needed fixing. I tried download/bypass filters but it did not put a file in the download folder (I think in the old days it would put a broken rar file and I'd just use quickpar to fix it).
So, I turned on autopar with aggressive assembly and it worked fine for one file but not the other. Got a generic error saying there were not enough pars, too many broken rars, or password missing. Fortunately it assembled the bad rar file and Quickpar fixed it just fine.
How do I assemble incompletes without turning on autopar?