The error is pretty simple: YUI Rich text editor module disables YUI Menu module.
Here is how I discovered this (may be it is my fault...):
1. I installed and enabled YUI Menu. It was working OK.
2. Then I uploaded YUI Rich text editor, opened admin modules page, marked it to be enabled, and pressed a button to save the changes, when the page reloaded the YUI menu was gone O.o huh?
All other menus were disabled so I had no choice but to disable YUI Rich menu editor... And then everything was OK again (YUI Menu reapered)...
Please fix this a really would like to use both modules in my site.