Aparently the problem was with the openssl library as installed by brew. [Or at least, this, and the following, is my reading of the situation]. Namely, openssl as delivered by brew is not signed, which caused the apple security framework to block it.
After a wastefull hour on the different stack exchanges I finally did the opposite of what one person suggested, I unlinked openssl (which I guerss is a variant of what you were suggesting)
Code:
brew unlink openssl@3
This gave back the ssl provided by the system: LibreSSL, and now things work.
Still, there were some issues (bugs?) when using the plugin: Only two of the three acsm files where converted to epub. The third simply did not work, so I went the old route via ADE and then importing to Calibre.
Big thanks for your help and for the nice plugin!