FWIW it works for me, but I did only a small backup and ran BackendTool and BackendTester.
Feed those a target URL similar to what Export As Command-line gives you.
Someone else is having mysterious S3 problems, and you can see the methods and talk here. Running some of those tests can see if your issue is similar. It’s possible these need dev help.
Verifies integrity of a backup. A random sample of dlist, dindex, dblock files is downloaded, decrypted and the content is checked against recorded size values and data hashes.
so even though the test sample is somewhat random, is everything selected failing that way?