Details are documented here. The after-backup verification is the same:
The TEST command
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.
<samples> specifies the number of samples to be tested. If “all” is specified, all files in the backup will be tested. This is a rolling check, i.e. when executed another time different samples are verified than in the first run. A sample consists of 1 dlist, 1 dindex, 1 dblock.
I suspect it’s smart enough to not repeat a file, so some types of files may go fully tested before others.
Regardless, this level of testing is going to be rough on your capped internet connection. Beyond really looking at the file contents, there’s always a file listing check (with size), but it wasn’t sufficient before…
Unless you deleted your entire Duplicati databases area, look in About --> Show log --> Stored for error.
Not very helpfully, I think backups that have no result statistics (because they didn’t finish) don’t make a typical entry in the logs at <backup> --> Show log (which is where GUI error
Show seems to always go).
There’s nothing special about your particular backup error, thus your log issue is probably the usual one.
How the error came about originally is a good question, but can’t be studied because backup is deleted, however even if it still existed there is possibly a closer look at history needed, and ideally debug logs…
If it can be reproduced, then it can be looked at closer. At this point, there’s probably not much to look at.