I am also having this problem -- Undo is permanently grayed out!! Brand-new MacBook Pro with 16GB RAM running 10.9.4 and using CC, not upgraded to CC2014 yet. On my previous machine I was running CC on 10.9.3, and did not have this problem. No font management software used, no secondary storage devices. Trashing the prefs in Home > Library >Preferences > Adobe InDesign > Version 9.0 > en_US does nothing. Rebooting the mac solves the problem temporarily, not sure how long it will last.
Adobe, this is appalling, please throw more engineers at the problem and find a fix!