After a bit deeper research, here is the temporary solution (you won’t be happy):
- take a deep breath and deal with the fact that WD MyCloud is a new, still unreliable technology and while WDStaff will do their best to help, more than likely it’s going to be a waste of everyone’s time. Likely, the “corresponding teams” will stay on hold until you give up. And even if the don’t, at the end you may have to send your hardware in for professional file-recovery (based on my experiences with WD, it’s likely they will help) so that’s only an option if you must have those files back, no matter what. In our case it’s development records of a fairly known software and some test results that are very important but we decided we can live without them. My solution is a drive format and sending it back (WD at least is very good in that part, refund/replacement) and getting a MyBook Duo until end of 2017. Hopefully, these “bugs” will be fixed until then.
Not being sarcastic, this is the only way if you don’t want to pull your hair for weeks. Get a hot tea and a new drive, WD drives are really good, this MyCloud thing is just not as reliable as it will be one day 
SOLUTION 1: Let it go
SOLUTION 2: Send it to WD
SOLUTION 3: not ruling out a solution from WDStaff it’s just very unlikely at the moment
