https://moodle.org/plugins/pluginversions.php?plugin=block_rss_plus
That plugin has not been upgraded for the version of moodle you are about to upgrade to. (Though I suspect updating it would not be that hard)
Re: The block rss_plus does not define the standard capability block/rss_plus:myaddinstance
by Marcus Green -
Number of replies: 0
According to this link