I don't know what might be the problem. Make sure you have foo_cad plugin
installed that is required by Rainmeter. I am using AIMP as my music player but I have installed the latest versions of foobar2000 and Rainmeter with cad plugin and it works fine. Maybe if you do not have cover images and foobar fetches it from net and Rainmeter fails to display it from some reason.
Please try some other Rainmeter skins to see if the same problem occures with them too. Hope you solve the problem.