So what Puffrfish also said before - you can not add a plugin.
But when youy create a server with a exist server type and then go with SFTP to the server files and replace the server.jar with a spigot variant it works.
Solution: After pinpointing the problem (404 error) I figured that my Nginx "pufferpanel" config was invalid. The error was due to the fact that in the "location /assets" block I had the following: "try_files /app/&uri =404;" instead of "try_files /app/$uri =404;". After changing the & to $ and then reloading Nginx everything worked as intended.