Downloaded many .apks including the 768x1280 ... all of them stopped after start up.
More information is required to make a meaningful diagnosis of your issue, but I will try to address what is the likely and obvious cause of this without you having to do so.
If you are running Android 4.4.- or higher, you will not be able to run Rockbox on your device, at all, period.
If you are running Android 4.3 or lesser (If you are, why? Please don't*.) and have manually elected to use the ART runtime engine over the Dalvik runtime engine, you will also not be able to run Rockbox on this device without reverting to the Dalvik runtime engine.
[Saint]
(* - I will clarify this position, if desired, upon request)