If the piwik installation is directly located at the root directory (piwik.example.com/ instead of example.com/piwik/), the plugin duplicates the / between host name and path, as

• the trailing / of the "Piwik Server" is enforced
• an input for the "Piwik Path" field is necessary -> /

This leads to URLs like https://piwik.example.com//piwik.php.

While everything still works, it is a bit confusing. See e.g. https://chef-ci.typo3.org