Fatal error: caused by Avatar Plugin

Avatar plugin not avaiable in the list.

I've had Avatar plugin installed on a multisite set up for some time and all worked fine when testing. I recently tried to activate Blogs MU-Child theme and the site broke. This error message was displayed on screen:

"Fatal error: Class 'WA_Widget_Avatars' not found in /public_html/wp-includes/widgets.php on line 413".

I located "WA_Widget_Avatars" to Avatar plugin and deleted the plugin. This made the site work again, but now without user and blog avatars.

I have not found any other discussions about this, so need some tips on how to fix it.