Can someone help me with this issue please?

It was my first time installing DR800+ in my DR800SG. (I haven't installed v1)
It seems like DR800+ v2. is successfully installed in my device(the new interface is properly applied!), but error keeps occurring:
Whenever I try to open a pdf or epub file, it doesn't open. No "please wait" sign and no flickering. Sometimes there is no response at all, and sometimes it seems to slow down or even stops working. It didn't matter what pdf or epub file I tried.
Strange thing is that html files and handwritten note files open without a problem.
P.S.: It's really amazing to see what you guys have achieved to make this device better.