Official comment
The first issue (export / playback) may be due to a low quality USB drive. What make / model are you using?
As for the "1/8th" loop -- that's probably because you're not in vinyl mode, and what you're hearing is what we refer to as "stutter cue." It's simply repeating the 1 frame of audio where the play head is currently positioned. This was the only way to cue a track on a CDJ before vinyl mode was created, and it remains as a legacy feature to this day. To prevent that from happening, simply enable vinyl mode.
Is there a reason you haven't yet updated from rekordbox 4.5? We're currently at version 5.4.