Windows 10 Enterprise 1803, Acrobat Pro DC 2019.012.20040, Firefox 69.0, Acrobat Add-on 18.0.10

Hi, when I try to convert a web page with the Acrobat Add-on in the upper right corner of my Firefox browser, I get an error message in Acrobat: There was an error opening this document. This file cannot be found. I cannot do anything else.
In Chrome I can convert to PDF without any problems.
Thank you very much for your help!