Startup failed Tiny Tiny RSS was unable to start properly. This usually means a misconfiguration or an incomplete upgrade. Please fix errors indicated by the following messages: PHP UConverter class is missing, it's provided by the Internationalization (intl) module. You might want to check tt-rss wiki or the forums for more information. Please search the forums before creating new topic for your question.
Лечится установкой пакета php-intl
apt install php-intl systemctl restart php-fpm.service
А на какую версию обновлялись?
до последней. Из гита.