I had the same Problem -- blank Screen when trying to upgrade from 5.0.14 to 6.0.0a on a QNAP TS 259 Pro+.
It worked after setting
eeaccelerator.enable = 0
in the php.ini.
And even after successfull upgrade, OC 6.0.0a works only with eeaccelerator = 0, setting it back to 1 leads to a "page not found" or to a blank screen. So it very much seems like an OC bug handling the php accelerator.
Oh, sorry - all the best to all of you for 2014!!!