I have reported this issue previously. It is not supercritical, obviously, just bothersome. When I drag a file into the Plex, this dialog box appears but remains after the file is comfortably in its resting place.
I wish I can report that this happens 100% of the time, but for one strange reason or another, it does not. Furthermore, on my laptop, it doesn't seem to happen at all (but the laptop gets much less use). This has me scratching my head as to what might be different. Both computers are Windows 7 64-bit, one with 6 GB of memory, the other (desktop) with 12 GB of memory and a superfast processor.
customized Desktop PC (Haswell i7 4770 3.4 gHz with 32 GB RAM), Windows 10 Pro 64-bit, 256 GB SSD, and a Microsoft Surface Pro 6, Windows 10, 512 GB SSD, 16 GB ram, i5