The WordPress updater and the pluginlist doesn’t work anymore and either loads forever or shows an error about files that aren’t there… And setting WordPress to debug mode didn’t create any logfiles.

/wp-admin/update-core.php
and
/wp-admin/plugins.php

To fix this in my case i had to delete the W3TotalCache plugin manually and reload the page.

For this i had to delete these folders and files:
/wp-content/plugins/w3-total-cache/
/wp-content/cache/
/wp-content/w3tc-config/
/wp-content/advanced-cache.php
/wp-content/db.php
/wp-content/object-cache.php

This fixed my wordpress admin.