"You have too many pending edits. Further edits cannot be submitted until prior edits have been approved." error displayed in a separate page

As you confirmed in the comments, you loaded the post you were trying to edit before you reached your edit limit, then after submitting suggested edits to other posts in other tabs or windows, you tried to edit this one.

I do believe that whether you can suggest an edit to a post is updated in the WebSocket pings, but that had likely not loaded yet when you tried to edit (or the connection was closed somehow). As such, clicking it sent you to the post editing page (/posts/[x]/edit), which loaded with the plain text error message instead of the editing form.

Just tacking on an official answer that this is pretty much status-bydesign.

There are a couple of scenarios where this can legitimately occur:

1. The window is opened, suggested edit is raised elsewhere that hits cap, you click the edit button on the original page.
2. If you middle click to open in a new tab, we actually bypass the inline JavaScript handlers.
3. You manually navigate to the edit URL.

We try to load the popup notice on the client, but since client side stuff can easily be avoided as well, we also verify on the server when the edit route is loaded to do one last check before rendering the edit page.