Trying to ugrade to 2013.06 I was getting a lot of "Could not get instance of datatype: xxx" errors when editing content classes.
Reverting the patch https://github.com/ezsystems/ezpublish-legacy/commit/1e27fa165cb20e26ae63ac3ccf621f0eee277356 seesms to have fixed the problem.

See also http://share.ez.no/forums/install-configuration/ezp-2013.6-could-not-get-instance-of-datatype-problem