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Thanks for your answers. I fixed the problem: It was about the UTF-8 compatibility. There was a hyphen in the online bib tex source (the one in the first bibtex entry) that was not supported, I just put in a 'normal' one and now it is working.

Hello,

I am rather new to Latex and Bibtex and trying to create my first document. I habe a *.bib file including two references. When I cite the second one it works perfectly fine, but when I put in the first one (in the same place) I get an error message Citation `frid2019' on page \thepage ...