I have read a bunch of issues on this forum about upgrading k2 beyond 2.6.9 and having save issues. I too have this issue. It has something to do with the class.upload.php file duplication. I decided on a test duplicate site I would uninstall k2 and reinstall it to see if that fixed the issue. What I got back was crap.

Fatal error: require_once(): Failed opening required '/var/www/vhosts/testsite.com/httpdocs/administrator/components/com_k2/lib/class.upload.php' (include_path='.:/opt/plesk/php/5.6/share/pear') in /var/www/vhosts/testsite.com/httpdocs/plugins/system/rebuild/rebuild.php on line 16

When I look at the ftp for the site the com_k2 is gone, but now I can't load my administrator site, nor does the frontend work. What is this error and how do I fix it. After fixing this, how do I correct this issue on my live site?

On the live site I have already tried installing k2 rather than upgrading and it doesn't fix anything.
Before uninstalling this I looked through the source code of my item page to see what else was loading to cause this duplication of class.upload.php and there was only your script and JCE. JCE is up to date.

So where before this install and on my live site where I was getting a white page and error stating that class.upload.php was already being used, so duplicate, installing 2.9 in the live site, then going to the rebuild.php file and changing the JPATH_SITE.'/administrator/components/com_k2/lib/class.upload.php' (this line is from the rebuild .php page) to wherever the new file is, should fix this? Do you know where the new class.upload.php file is for directories?