There are 2 solutions
- Ask your webhost to increase the max_execution_time in your php.ini file, for example to 30 seconds.
- If you can’t change this setting, you can alternatively disable the feature to combine together CSS and JS files. To disable this feature, edit your config/config.ini.php and add the following:
[Debug] disable_merged_assets = 1
Then, delete your browser cache and refresh Piwik: after a few seconds, it should display correctly again!