I’d suggest testing a problematic file locally (copy to sdcard or a usb stick).
If that works then it confirms a network bandwidth issue.
If it fails then it’s a file/pi/player issue.
I think – but may be wrong, that the Apple iPad power supply, while offering up to 5.2V can dip to 4.75V, which makes it unstable and not suitable for a Pi.
Do you see the rainbow icon in the top right corner at all?
Also test with local playback as popcornmix suggests to rule out a network problem
My iPad charger dips to 4.3v attempting to run a Pi 2, so not usable!
There are several models of iPad charger so some may be better than others, but they are not recommended. Get a power adaptor designed specifically for the Pi.
I have a 5A powersupply and it became apparent that with an update, the voltage dropped below 5V but that was a problem with a very thin cable I used to power the pi.