The table looks like it has about 55 other things that could give you the 400, but good luck finding the one. Probably some logging (e.g. About → Show log → Live → Information or testing is in order to know what operation was being attempted at the time, then try doing it in Duplicati.CommandLine.BackendTool.exe. Export As Command-line can give you a URL to use, but you might want to edit it to a different test folder.
Duplicati.CommandLine.BackendTester.exe can do a slower but automatic test if you prefer that method.
There is no special support, but isn’t this designed so that your bucket setting (which is what?) can set it?
The program does need to live within the immutability provided. Do you intend to let backup grow forever?
The minimum would be to keep all versions and set no-auto-compact, but maintenance and upload errors might sometimes want to clean things up, and sometimes this may need deletion, if a window can be had.
Here is an example where the contract prevented disabling immutability, but is it administratively possible?