DFPlayer Mini Help with Wrong Stack Error

Greetings! I have been working on a foot pedal that will act like a stomp box mp3 player on stage at our church. I built my first prototype several months back and ironed out all the kinks. Since then, I have been asked to make more by friends and have been having issues with the new ones I am making. The DFPlayer mini will initialize, but will display the "wrong stack" error as soon as I try to play any files. It will then "Time Out". I have not been able to find anything online to help understand what this error means and what I need to do in order to fix it. I am using an Arduino Nano, DFPlayer mini, and the DFRobotDFPlayerMini library.

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The old version works fine. What is different in the new version?

There are many DF Mini libraries out there. I'd switch to a better one like the DFMini_Fast library. Just type in DFMini in your library manager and several will pop up.

The lack of a decoupling cap for the DF Player Mini is a potential issue.

Not having both ground pins connected is another.

A 7-segment display with a single 110 ohm cl resistor (electrically) and a DFPlayer with built-in 3watt amplifier, all powered through the onboard 5volt regulator also sounds like a potential problem.

What is connected to that mini jack output, and what voltage is connected to the DC socket. Would be easier on the Nano if you power it with a 5volt cellphone charger, connected to the USB socket. Leo..