How to fix MakerBot Error 81 due to false-positive filament jam
The printer model is MakerBot Replicator Mini - it's more than 5 years old I think. the print cartridge is a "Smart Extruder", not a "Smart Extruder +" which fixes this issue I had. This issue is discussed here: https://www.theverge.com/2016/1/4/10677740/new-makerbot-smart-extruder-plus-3d-printer-ces-2016
This Error 81 in my case was caused by the "encoder wheel" getting stuck, which triggered a false error. This is discussed in the MakerBot support page, but the page doesn't tell you the right way to fix it (at least not for me): https://support.makerbot.com/troubleshooting/makerbot-replicator-mini/error-codes/error-81_10672/2902
See next comment on how to fix it (i hit a character max)