rss: bot does not work for some feeds

[LuckyTurtleDev]

Describe the bug
For some rss feed the rss-bot does not work.
It does simple send no message. But other feeds does work.

To Reproduce

• create new unencrypted room.
• add the rss bot integration.
• subscribe https://warnung.bund.de/api31/mowas/rss/053340000000.rss or https://maintenance.rz.rwth-aachen.de/ticket/status/messages/feed.rss
• wait for an update of the rss feed

Expected behavior
A message if the rss feed is updated.

Additional context
I was only able to reproduce this with rss feed in german language. May it happen, because the rss feed does include chars like äüöß?