Hi Johannes,
Yes, if the RSS format is incorrect then any reader would display the same.
It is indeed possible to hack around with the code to fetch the date from the feed, but it would be just a workaround.
If possible please check with the feed owner if that format can be changed. Otherwise let me know I will dig through the code and see if something can be done. You might have to keep the plugin without updating.
Thanks,
Aakash