9
As has been said before, though perhaps not in these words, there may be another reason why you have blank pages and you should try and fix that issue before you move on to trying to update (IMHO). What path did you take to upgrade? Did you run any upgrade scripts when you upgraded?
Did you go from 2.0.13.2 to 2.0.15? If you did the latter, you should understand that it is probably not good to bypass releases when upgrading. It's best to upgrade release by release eg. 2.0.13 to 2.0.14 to 2.0.15. Also, make sure to run any attendant upgrade scripts. There's a script (in the upgrade folder) in 2.0.14 that you must run to upgrade from 2.0.13 to 2.0.14. Likewise with 2.0.15. If these upgrade scripts aren't run, then significant changes that occur in the database will not be made. And, in recent releases, significant changes were made. If those changes aren't made, it heightens the possibility that you're site won't run as is.
Also, if you upgrade without finding out what's wrong you exponentially increase the risk that you will lose data. If the database changes and you try to reinject the old database data, you'll likely cause yourself greater amounts of misery. The changes may not be that great. On the other hand, they may.
But, that's just my two cents, pesos, etc.
MMM...It tastes like chicken! 
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