I've got a Plone 4.1.5 in ZEO, and when I activate RAM Cache in Plone Caching, the diazo theme is no longer applied.
I have checked the logs, there is nothing.
I'm running plone.app.theming-1.0b8 & plone.app.caching-1.0.3
It might be similar to that bug: https://dev.plone.org/ticket/12038