After upgrading I got an error page saying first
No entry or class found for 'Piwik\Plugins\Actions\Menu' and then
No entry or class found for 'Piwik\Plugins\UserCountryMap\Menu'. I tracked it down to tmp/cache/tracker/eagercache-301-ui.php that contained
[...] 'PluginActionsMenuPiwik\\Plugin\\Menu' => 'Piwik\\Plugins\\Actions\\Menu', [...] 'PluginUserCountryMapMenuPiwik\\Plugin\\Menu' => 'Piwik\\Plugins\\UserCountryMap\\Menu',
I know you can work-around the problem, by setting
enabled = 1 in the
[Development] section of config/global.ini.php
Anyway, I realized what is going on. Before upgrading I forgot to delete all files from my earlier Piwik 2 installation. That is a user error, but please don't close this as a "won't fix". I think a lot of users can do this mistake, and it's hard to find the work-around - and all you get is the error page.
Could Piwik use config/manifest.inc.php when generating eagercache-301-ui.php to avoid this problem? I don't know. When you finally get to the filesystem integrity check (which isn't displayed if use the work-around above) you see the files that are leftover form 2.x and should be removed. Easy. I removed eagercache-301-ui.php so the system check page displayed.
We have recently fixed this and we now show a list of files that should be deleted based on manifest etc. :)