fetch_feed function works on template index.php but not in plugin/wp-admin

The error suggests the problem– ABSPATH and WPINC are not defined. That is why you are seeing ABSPATHWPINC/feed.php in the error. PHP is trying to make the most of the undefined constants and is treating them like strings. It is easy enough to demonstrate this by echoing a string like IAM.NOT.'defined'.

This is likely because you are loading rss-plugin-scan.php directly which means the WordPress core isn’t loading to define those constants. You should be loading that file in a way that also loads the WordPress core, or the parts that you need..

I don’t know how your plugin works, or what it ultimately needs to do, but you may want to load that file over the AJAX API, or by doing something like @Rarst explains here.