I tried applying to only $error and only $old_error, but both appear to be needed.

Log entries:
E_NOTICE (8) 	classes/urlhelper.php:464 	Trying to access array offset on value of type null
1. classes/urlhelper.php(464): ttrss_error_handler(8, Trying to access array offset on value of type null, classes/urlhelper.php, 464, [)
2. classes/rssutils.php(464): fetch([{"url":"https://some.url.rss","login":"","pass":"","timeout":15,"last_modified":"Sat, 31 Aug 2019 15:22:31 GMT"})
3. update.php(235): update_rss_feed(732, 1)

 * cleanup update_rss_feed() a bit, use ORM

different agent headers for whatever reason, example:

Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML,
like Gecko) Chrome/88.0.4324.192 Safari/537.36

Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like
Gecko) Chrome/88.0.4324.104 Safari/537.36

seems to be related, at least, to App.postOpenWindow() hack.