I have and had this problem across any and ALL themes, after activating an RSS feed widget. I removed each and every (all!) widgets. Still had both the subject error plus a “cannot update header” error.
Another forum solved the second (header) issue,
but still stuck with the Warning: Missing argument 2 for wp_widget().
SOLVED it by renaming (erasing) the new widget’s folder from the wp-content/plugins folder
same problem, two errors AFTER I activated a RSS widget from a guy who asked for money for it because he’s only human…
Partial fix:
Your link to the blog that said ffix the wp-config.php file solved the header error because it had removed the final ?> from the config file
Still have the error “Missing argument 2 for wp_widget()” everywhere on site!
