Ah, yes, that seems to be the Certbot 3.0.0 update giving you issues, not the njalla API or plugin itself.
You could revert the Certbot snap to 2.11.0 I guess. Someone on a certain issue on the Github Certbot repository used sudo snap revert certbot --revision 3834 to revert the Certbot snap to a pre-3.0.0 version.
Also, perhaps you could message the author of the plugin (chaptergy · GitHub) and point them to Certbot 3.0 Could Have Potential Third Party Snap Breakages and Architectural Decision Records · certbot/certbot Wiki · GitHub.