Theory seemed to test out, however after theory comes waiting for fix and release, so avoidance such as described in the “Verification errors” link will have to hold for now. Snapshots on Ubuntu are a little unlikely because I think the default setup doesn’t set up LVM (do you know how you set up?), and if so, then using Screen 3 “Source Data” changes such as unchecking sources or adding Filters can keep Duplicati out of attempting to back up its own rapidly-changing database journal. While it’s sometimes useful to backup a Duplicati DB when it’s inactive (e.g. using a different job) to ease disaster recovery, active DB is little help.
As a Docker side note (and I’m not an expert at all), are you keeping Duplicati’s database in its container? Generally I thought one wants persistent data outside the container so the container can be replaced, e.g. whenever time comes for an upgrade to a newer container. The other option might be to let Duplicati just upgrade itself inside its old container using the same mechanism it would use if it were not in a container, however IIRC there are Docker users in the forum who favor the container-keeps-no-persistent-data way.
If you’re willing to look in your DB after this error, you might see this sort of result (from my test last night):
- 2019-08-21 23:02:10 -04 - [Error-Duplicati.Library.Main.Database.LocalBackupDatabase-CheckingErrorsForIssue1400]: Checking errors, related to #1400. Unexpected result count: 0, expected 1, hash: iZFxgvrWfC+8CJIwcfhI+DEeLAkD4HHvwtZF/TJ9baw=, size: 276, blocksetid: 8, ix: 22, fullhash: K8w3a6PSGLR2DsXmDpkaaD7rkQveEJyE1+cxbcS4rIk=, fullsize: 22804
- 2019-08-21 23:02:10 -04 - [Error-Duplicati.Library.Main.Database.LocalBackupDatabase-FoundIssue1400Error]: Found block with ID 12 and hash iZFxgvrWfC+8CJIwcfhI+DEeLAkD4HHvwtZF/TJ9baw= and size 272
This had 1024 byte blocks, so 22804 bytes usually means 22 * 1024 = 22528 bytes plus 276 at the end.
It expected the block to be 276 bytes at the end of the file, but actually blocks arrived as 272 and 4 bytes. Surprised at the lack of the expected block, Duplicati complained, talking a lot about the numbers above.
Blockset 8 never got its 23rd (partially filled) block, because its row insert inserted 0 rows (instead of 1), because it couldn’t find the block it expected (topic of error messages), because the block got split up…
Test script writer.bat was:
@echo off
:top
echo * >> writee.txt
goto top
You shouldn’t really need to know all this, but you did ask what the errors meant, so I explained a lot. 
Another reason this is posted in some detail is so I can point to it from other posts, or a Duplicati Issue.