Log into NVR via web browser → Playback tab. If same error appears → issue is on device side (firmware/HDD/permissions).

Video playback requires a sustained, high-bandwidth, low-latency TCP connection.

This is the #1 software-based cause. Smart PSS asks for playback "from 2:00 PM to 3:00 PM." If your NVR thinks the current time is 1:00 AM on January 1, 1970 (a common default after power loss), it cannot locate those files.