I'm using version:

roundcubemail-plugins-kolab       1:3.3~dev20160111-0~kolab1.2

When manticore isn't available or has been disabled/not loaded (like not available on debian yet) kolab_files generates a javascript error when opening the calendar.

ReferenceError: manticore_api is not defined
kolab_files.js:199:3

The code block in question looks like this:

function kolab_files_init()
{
    [... lots of stuff ...]

    manticore = new manticore_api(manticore_config);
};

I've just realized the core already looks a bit different due to WOPI integration, but in general: We can't call manticore_api isn't loaded/available