I'm having a similar problem with my return, coincidentally also at the DEN airport location. I returned my car Sunday evening; the agent scanned the car and said they'd email me a receipt. (Shame on me for not asking for a paper copy)
It's now 48 hours later and there's no receipt; I checked my app and online and it's unable to generate the receipt for me. I tried the 800 number and was told that while they show that the rental is closed, 'the location is still processing the rental and it could be another 24 hours before the receipt is generated.'
Seems pretty ridiculous to me; I tried calling the DEN location directly but no one answered.
Anyone else encounter anything like this?