Hi Marvin!
I found out that I can only open the document in the library when I check it out first (I had following Settings):

I can open now the document within the workflow as I changed the "Auschecken erfordern" button from "yes" to "no" (see picture below). But I would like to set that the collaborators have to check out the document first, so that they don't accidentally edit a document (e.g. by accidentally clicking on it) and the document is then back in draft mode and has to be approved again. --> but then the worklow fails again & no one is able to open the document.

Maybe this additional information is also helpful:
- when the setting "check out is required" is on "yes" the workflow is triggert by checking in the document
- when the setting "check out is required" is on "no" the workflow is triggert by "publish main version" (see picture below)

Best regards
Susi