I'd recommend opening a Support case. Typically snapshot issues are due to:
The required port is not allowed upstream - TCP 30001
Requesting the image immediately after the request
It can sometimes take 5-10 seconds for the image to actually be downloaded from the MV, so make sure you give a 5-10 second buffer between the snapshot request, and attempting to view the image.
Requesting an expired image
The MV will only store the last 10 snapshots, requesting an older snapshot will result in an error 400 (like your screenshot)
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