Exact same issue here, but with the Tapo 325E. 

Firmware up to date

Wifi router rebooted

Camera rebooted

Removed/re-inserted microSD

Streams live view fine.

Complains about unstable network when scrubbing through video.

Queueing up download of videos adds to the queue in the Download tab under "Downloading video..." progress bar, but shows up as "Failed". Clicking the refresh icon tries again with a progress bar for a few seconds, then fails.

In the camera settings, network connection shows full green WIFI bars...

I just moved it to ethernet instead of wifi, super solid network now. Still fails.

I figured it out. The camera stopped loop recording a few months ago but kept taking snapshots. It still showed up as loop recording though. Anyhow, it seems that if snapshots of a time window exist, but there's no actual video of that time window, it will show you the option to download a video or scrub/preview, and then fail the download/preview because it doesn't exist. It will proceed to blame your network and not its own failure to maintain loop recording. 

In my opinion, tp-link should have way better error handling/reporting in the firmware. 

@OP I bet if you take the SD card out and put it into your computer, you'll see something similar. Old videos, but up to date images in the snapshots folder.

I formatted my SD and started from scratch, we'll see if it loop records as expected...