Calling someone to your room overnight makes it so that money cannot be collected from the vending machine the following day. Players need a better money-making option than vending machine.
Viewing post in Odd Inn Onsen v0.3.1 (Bug Fixes) comments
oh so thats what caused the issue with the vending machine. Ran into it as well but had no idea what caused it.
Also if you want to skip having to farm money via the vending machine, just go and upload your save to an online save editor and adjust the amount of money you have. And in case you didn't know: Buying new sorts of ice cream for the vending machine increases the daily income.