I have a feeling the Amazon S3 feature is buggy - I haven’t tried it myself but my hunch is based on feedback by couple other users (one of EX2 and one of EX4 - and both EX2 and EX4 use the same codebase as Mirror’s, so whatever problem is plaguing the Amazon S3 support on your Mirror is almost certainly an issue on those models as well). There are certain issues that WD takes forever to fix in their firmware, if at all…and given how few people might be using the S3 backup feature, I doubt it is even on their radar. The best way to deal with these bugs is to definitively prove to their tech support that this is really a bug (like I did with an FTP bug that was present on my EX2 and on Mirror and they fixed it after five months from when I reported it in their firmware release last week).
This is where you can find more about those other users - my advice - band together with others in your shoes and be in touch outside the forum and build your case if you want the issue to be addressed by WD at all. Ad oh, you do need to open a trouble ticket (which WD has a bad policy of closing even if an issue is unresolved if there is no activity on the case for one week) → http://community.wd.com/t5/WD-My-Cloud-EX2/Problems-with-Amazon-S3-Jobs/m-p/759676
