For me, it is as other users wrote in similar threads - having smithed items in inventory causes the crash reliably post 1.5.7 hotfix for me.
My saves are not corrupted. When I sell or drop all crafted items in inventory (those that are equipped on the character can remain on character, so for me that was just a few dozen javelins), I can attack looters just fine in my existing saves and can continue my campaign.
Reliable repro on new campaigns as you decribe - run for Poros, smith something out of your starting cash, seek out looter party -> crash. However, only when you have that smithed thing in inventory....
Did you try dropping everything from the inventory in your existing saves and try attacking the looters to see if that works for you too? Because putting that smithed stuff into your fief's stash is maybe a nice workaround to continue your save until this gets fixed. nd you can use smithed weapons just fine, as long as they're equipped, not in inventory, so it also doesn't render smithing itself useless.
At least for me. Maybe I'm lucky.