I have stated that the problem systematically disappears when the PC in question is disconnected, or when the laptop is online. As far as proof goes that it's network related, I don't see what more you could want. As stated in the other thread, I've already tried Event Viewer—nothing relevant there. Procmon: I did what you said, and it just triggers a crap ton of explorer.exe entries, many of which are mp4 related, but I have no idea what to do with them. Wireshark: Just clicking an MP4 indeed triggers a series of SMB/SMB2 packets from 192.168.0.91 (PC in question) to 192.168.0.94 (laptop) and back, and this doesn't happen with any other file format. If I do it while the laptop is offline, I immediately get a mix of TCP packet losses and ARP packets saying "Who has 192.168.0.94? Tell 192.168.0.91". This confirms what I already knew, but still doesn't tell me what the cause is.