PEBKAC, Sorry this ist not Helpfull.
Show the Thread,. You See Original Firmware Work. Here is no defect oder SD-Card inkompatible issue, The Original Firmware work. You can not ignore that.
Now can a Problem with the MP3 Files. Ok
Foobar / lame mp3 v2 MP3. Make from Flac. Original Firmware Perfekt. No language issue. No Play issue.
On Rockbox freez MP3, freez Flac from SD-Card
Or Rockbox is not SD-Card compatible. With Original Firmware the Card Work. I can not buy tonns of SD-Cards to see witch is work with Rockbox.!!!
And disses are not Helpfull. Is not resolve the Problem.