You are correct about starting the server from the parent folder of your prefix. Please check if your prefix is set correctly. You can check the global prefix by running command:
beat config show.
Since you set the path manually as please check the .beatrc file in the parent folder of your prefix as well.
Also please check if you can see your list of experiments in the terminal by running command:
beat exp list
This problem may also happen if there is an invalid block (algorithm, toolchain, etc) in your prefix folder. First try the above commands and see the prefix used by beat.