I just found when trying to use IFTTT that my blog's wordpress feed seems to be faulty: https://validator.w3.org/feed/check.cgi?url=https%3A%2F%2Fwww.kallmeyer-naturheilpraxis.de%2Ffeed
There appears to a strange element in the beginning.
I am by no means a programmer but this is a great point to start learning php. Unfortunately after checking most of the wordpress php that they are as they should be.
So now I have no idea where to look else. I did download all php files locally and tried to search for this element - nothing.
It would be nice if someone could put back on track where possibly to look for the bug.
Thanks.