Can't add site, The method 'isPluginActivated' does not exist or is not available in the module '\Piwik\Plugins\CoreAdminHome\API'. (no replies)

Running Piwik for years, now on version 2.9.1 I believe.

I need to add a few sites. When I go to the website manager page, a red error appears briefly that says:

The method 'isPluginActivated' does not exist or is not available in the module '\Piwik\Plugins\CoreAdminHome\API'.

and I am unable to add any sites. Any workaround?