I can confirm this problem as well.
Having trying out the iOS-platform for a while I went back to the Android platform. So I logged into my google account on a new S10 plus, it restored my apps. Poweramp included.
I copied over my folder with music and noticed quite quickly the same problems that some other person mentioned. The songs skips and I get something like "could not be played" a lot, can not remember the exakt phrase.
The same thing others describe is also hitting me, the thing where I can not select folders but only choose internal storage.
I am not comfortable installing apps from any site, this site included, and would prefer to get a fixed version from the google play store.
This must be a permissions thing or something with how the folder is mounted for reading. I also noticed that sometimes even though it says that it can not play a file, if you continue to press the play button, it actually will play it. That could mean that there is an intermittent reading problem. It might very well also be an Android bug as well. As a developer you sometimes pull your hair trying to solve an issue when eventually it turns out it was an OS bug. Neither Apple nor Google are perfect in this.
Thanks for still developing this app, the sound settings are just leaps and bounds over other music-apps 👍🏿