Having the same issue while trying to add an item via frontend and getting the following warning;

Warning: require_once(/home/luminate/public_html/administrator/components/com_finder/helpers/indexer/adapter.php) [function.require-once]: failed to open stream: No such file or directory in /home/luminate/public_html/plugins/finder/k2/k2.php on line 15

Fatal error: require_once() [function.require]: Failed opening required '/home/luminate/public_html/administrator/components/com_finder/helpers/indexer/adapter.php' (include_path='.:/usr/lib/php:/usr/local/lib/php') in /home/luminate/public_html/plugins/finder/k2/k2.php on line 15

Joomla 1.7.3
K2 2.5.5

I'm getting the same error guys. I just installed the most recenet K2 software today with Joomla 1.7.3 and as soon as I try to create a category from the back-end I receive the failed to open stream: No such file or directory in... error.

Is this a stable version?

Many members of Russian joomla community have the same Error.
I can offer to turn off search plugin for K2 and com_finder.
But it's not right solution of problem

So Im not the only one with this issue,
I have been completely baffled by this. Is there not a previous version that is stable that could be used?

I have exactly the same problem .. just adding my voice for a fix, please!!

Well, I found that K2 works perfectly with Joomla 2.5 ... Joomla 1.7 is missing several things that K2 refers to. I started hacking Joomla 1.7 to get everthing working - realized after a while that I'm just doing a painful manual upgrade to Joomla 2.5 .. so I just installed 2.5 :)

To everyone facing this issue simply find the "Smart Search - K2" plugin and disable it. We are going to add an extra check when K2 is installed to skip the finder plugin installation under Joomla! 1.6/1.7. However since Joomla! 1.6 and 1.7 are not LTS releases, anyone who uses those versions is highly encouraged to upgrade to Joomla! 2.5.