Bump!
Same issue.
It started to happen when I upgraded Joomla from 3.4.8 into 3.5.1
I had Falang 2.2.2 installed.
I also upgraded Falang into 2.5.0 now, but problem still exists - k2 buttons doesn't work
Other joomla top buttons works OK.
K2 v2.6.9 (Developer Build)

I have another website with K2 v2.6.8 and Joomla 3.5.1
Just now I installed latest Falang 2.5.0 and it's K2 content elements (v2.5) and all works OK
- no errors when publishing or unpublishing k2 items or clicking top K2 buttons.
Weird

EDIT FIX:

Post by Stephane, Falang developer:
It's fixed in the next release of Falang > 2.5.0 , the 2.7.0 of k2 change the return of $id to array.
line 226 of plugins/system/falangquickjump/falangquickjump.php
you can add this
if (is_array($id)){$id = $id[0];}

just before the
//Load ToolBar
$bar = JToolBar::getInstance('toolbar');

I tested it and fix works OK.

Regards