Big Sur and FTP certificates

After upgrading to Big Sur, I am no longer able to upload changes to my websites directly from Rapidweaver. I have struggled for weeks to find out how to fix this. Today I saw that when I published to a folder on my disk, and then uploaded it via Cyberduck, I got an error message from Big Sur, saying that it didn’t trust the certificate of the ftp server. I could choose the option to “Always trust” this site, and then go through with the uploading. However, Rapidweaver do not bring up such an dialogue that lets me choose the “Always trust” option, instead the program just hangs on publishing, waiting for Godot.
What can I do with this problem?

What publishing method are you using?

I’d be more concerned with what’s wrong with the certificate? Who issued it?