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Has it been tested with S3 as primary storage? I assume it works, but better confirm it.

Note: I raised issue #39725 to add a few test scenarios that use the webdav_owncloud external storage. We already have a core test suite that has the environment all set up, so it should be "easy" to demonstrate some current core API behavior and then see that the various prohibited move attempts get a different/better response message with the code in this PR.

@janackermann PR #39772 add API test scenarios relevant to this fix. It has been merged.
I have rebased this PR and adjusted the 2 test scenarios that now give the correctly-adjusted message - good.

But there are 2 other new scenarios in https://github.com/owncloud/core/blob/master/tests/acceptance/features/cliExternalStorage/filesExternalWebdavOwncloud.feature that give a 500 error. Look for the issue-39550 tag.

Please have a look at those. Can the 500 status error be fixed here in this PR?
(And adjust the test scenarios so that they have corrected test expectations)

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Rebased - hopefully CI will still be happy.

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@janackermann I rebased and this passes - is it fine to merge?