It turned out it was no memory problem whatsoever – I used the WordPress Plugin “WP-Memory-Usage” and the memory usage never goes beyond 20MB, even on complex pages. My problem was of a different kind (which you solved in another support request): the Advanced Editor was disabled!
To enable it, open the page you want to edit (which shows the “Advanced Editor” button which won’t work), then, click on “Options” in the top right corner to bring down the options menu, check “Avia Layout Builder”, reload and voilà! Next time, the Advanced Editor will (hopefully) start working again.
I’m just repeating this, so all hail the Kriesi support team. :)
I have the exact same problem. Memory Limit was set to 64M for PHP and I increased it to the same amount for WordPress, as you proposed but it didn’t change anything. Deactivating plugins (PC hide pages and Akismet) also did not better anything.
Strange thing is: the Advanced Editor still works when used in the Portfolio. No JavaScript errors pop up in the log when Firebug is active. No Errors pop up in the webserver’s (Apache) error log.