Fatal error 3624 : iKey GetKeyCount()
This issue is to do with corrupt/invalid statistics.

A specific case where it can occur is mentioned in this connect item


  The Metadata of disabled Non-Clustered Indexes becomes invalid, if you
  change the keys of the Clustered Index, and this caused the failure
  that you experienced.


With the workaround being to drop the disabled indexes.

If the situation above doesn't apply to you then try and find out the specific table with the problem hopefully dropping the statistics on this specific table or both tables if you can't pinpoint the problem one should do the job (and then recreate the stats if required). 

If neither of the above work then probably time to open a case with Microsoft PSS.

Also you appear to be on SP2 + Security Bulletin MS15-058. There have been a lot of cumulative updates and another service pack since then. Possibly the issue you are experiencing is already fixed in one of these.