Check permissions. Android 11 changes stuff. Otherwise try clearing app data and setting up again.MXPlayer can't find custome codec file. Actually MXPlayer file browser doesn't show any file but just empty directories/folders. I've been updating EAC3 custom codecs for MX successfully many times before on my Nvidia Shield and everything was going well. I guess this problem occur after Shield 9.0 update.
Has someone came accross with such issue?