I live in an older duplex. I just recently attempted to clean out the dryer vent/duct for the downstairs unit and failed because I could not find where my dryer vent is outside. Is it possible that my dryer vent and the upstairs dryer vents are connected to the same outdoor vent?
Place a wet towel in your dryer, turn it on, and in 5 minutes look for the steam somewhere around your house.
It's possible an earlier repair/renovation blocked the dryer vent. See if you can trace the duct from the basement to see where it used to go.