Yes, such symlinks do exist (as I wrote under 2.). But as far as I'm aware, they only exist in the documents folder. And this one I have already excluded from the backup (for testing). All the other folders in my libraries should still be backed up, I believe.
Does the backup fail, before the exclude list is executed?
I tried to get an overview where else symlinks might exist without my knowledge. I tried
Get-ChildItem -Path "D:\documents" -Recurse -Force | Where-Object { $_.LinkType -ne $null -or $_.Attributes -match "ReparsePoint" } | Out-File D:\links_documents.txt
but this yielded no result (empty file). Can you give me a hint what's wrong here?