This indicates some server error.
First off, install v2.9.1 (dev) from
github.com/getk2/k2/archive/master.zip on top of v2.9.0. If this does not resolve the blank page issue, switch error reporting to "maximum" in Joomla's Global Configuration and check again to see which errors are reported as it could be some 3rd party plugin causing issues (for example).