I had the same issue, what worked for me, was renaming the stack.yaml file to stack_notrequired.yaml (or what ever name you want to give) probably deleting the file will also work. But I usually don't like deleting files.

The language server after this change stared working in the intellij environment for the Haskell hls plugin.

All other solutions did not work for me. I don't have an explanation as to why this is the case.