I a have a similar problem after update to v4.2. The blocks are visible in admin preview also in public siteaccess, so they do exist. But when i start to edit the frontpage i see only the thumbnails of the zones, no tabs, no blocks.... nothing. Also if i select a zone and press 'Select', the page is reloaded without any change. Thus i can't see the current blocks, nor i can create new ones.
The rewrite rule from upgrade procedure is present in my conf file:
Another problem that might be related is that i cannot import vie admin interface the ezjscore_extension.ezpkg. I can import and install all other packages mentioned here http://ez.no/doc/extensions/ez_flow/ez_flow_2_0_upgrade but this one i can't. When i press import, the page is reloaded without any change agian to import view.