Hi Martin,
Yes, this can cause this issue. When you upload the file to DropBox, it is automatically removed from your server - unless you disable that option from the DropBox engine. As a result, when Remote Control asks your site to give it a filename so that it can download the backup, there is no file and you get an error. If you disable the file downloads in ARC you should have no problem.
Moreover, if the upload to DropBox takes too long, it's possible that an error occurs (usually a timeout error). In that case, your server kills the backup script and sends ARC an error reply. ARC sees the error reply and halts the backup. However, as you told me you can backup from the back-end just fine, so there should be no timeout issue.
Nicholas K. Dionysopoulos
Lead Developer and Director
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