I figured out, that beyond a certain folder size, Poweramp cannot detect the music files. Resulting in incomplete library, with many songs missing. (The Search function was malfunctioning too.) The maximum folder size seems to be somewher between 17GB and 41GB.
I had this problem with a 41GB music folder on SD memory card. In the 41GB folder, the files were organized in subfolders by artist. So the subfolders were not so big, but still caused problem. I managed to solve all problems, when i split this big folder into 3 smaller folders, and put them into the root directory of the SD card.