There is a GP community forum question that discuss about GP 2015 R2 upgrade errors. As always, contributors have poured in their thoughts and possible solutions to fix these issues.
Read the question here: GP 2015 R2 errors during update & Fixes
I recommend this post to everyone who are planning to upgrade to GP2015 R2 soon. This may save a lot of time.
Thank you GP community for being selfless, informative and helpful.