Can you try to delete the files within the folder tmp/cache?

I tried to reproduce with PHP 7 and worked fine for me.

I tried that, unfortunately without any effect. But I took a closer look at the files in the tmp/cache/tracker folder because the files inside were generated when I call the Custom Dimensions entry in the menu.

I found out that there are two language-files, called piwikcache_Translations-en-{$hash}.php and piwikcache_Translations-de-{$hash}.php where {$hash} is a string like f2e5cd85c39e6c78f64c1a41d155f2ea691b19aa. In the English file (en) I found an array with the key CustomDimensions containing some phrases. But in the Eerman file (de) it's completely missing, which fit to the error message.

So I tried to switch the language in my piwik account from German to English and it worked! The problem seems to be that the CustomDimensions plugin wasn't translated to German, which I didn't expexted because all other plugins I'm using working fine with German.

Don't know if this is a mistake to be fixed. But as the dimensions are recommended as an alternative to the custom variables, it would be a good idea to translate the plugin at least for the final release to German I think.

Our translations are community contributed. If you would like to help, please get an account on transifex: https://www.transifex.com/piwik/piwik/

Nevertheless, missing translations shouldn't cause anything to fail. Piwik is designed that if a certain text is not translated to the currently selected language, it should fall back to the english default, and display that instead. Guess we need to investigate why that doesn't work in your case.

This is actually a bug and I will work on a fix asap. I thought I'm smart and simple pass all translations of this plugin to JavaScript as it is build in angularjs. Turns out Translator::getAllTranslations() doesn't return all translations and it wasn't so smart to do this :)

I just released a new version of the plugin and an update should be available soon. You might want to press "check for updates" in the Administration to get the update earlier.