I just have installed K2 to my Joomla 3.4 website. When I try to create a new item I get a
Fatal error: Class 'MVApp' not found in mysite.nl/public_html/plugins/k2/mvgallery/fields/mvgallery.php on line 52
What is the reason and (more important) how can I solve it??
Who can help me?

Hello Gert,

This is unrelated to K2. The Mvgallery plugin is causing this issue.
You need to address this issue to the plugin's developers.