Canceled: No client for fragment XXX (Ajax client enabled)

HI

I am getting Debug message 'Canceled: No client for fragment menu-local-tabs'

Enabled modules: Authcache Ajax, Authcache menu, cacheobject (for forms), Authcache flag.

routes rebuild, clear cache didn't help.

sometimes, I am getting No client for fragment f1 (flag).
sometimes the result was: Fatal error: Call to a member function handle() on a non-object in frontcontroller.inc on line 82

What could be wrong? How to Debug that?

Thanks