I have a similar/identical issue. My flow captures attachments from an Office365 account to Dropbox. The Flow works and fails with little apparent logic, except for this theory.
On an occasion where the Flow failed, the output contained the following...
{ "status": 409, "message": "File '/office365 mail attachments/hawkesbury council attachments/ATT00001.jpg' already exists. Use the Update operation to update an existing file.\r\n inner exception: Exception of type 'Microsoft.Azure.Connectors.Dropbox.DropboxAdapterException' was thrown.", "source": "127.0.0.1" }
My annotation in red, as perhaps the reason why it failed.
If this is the case, then the question is: How can I edit a flow to ensure that, rather than failing altogether, it saves the attachment with a new filename (eg ATT00001(1).JPG in the above example)