I have just managed to open a Query Tool in the Query Tool Workspace.

First, I disconnected from my database in a Default Workspace. Then I manually entered the Database and User names in the corresponding fields in the Query Tool Workspace, entered the password, removed all rows from the "Connection Parameters" section, and after all that, pgAdmin finally allowed me to connect.

Works the same if I don't select anything or clear the "Existing Server (Optional)" field and manually enter other info.

To be honest, this Query Tool Workspace feature is kinda disappointing. I did not achieve the expected results with it, and this "bug" with the connection just annoyed me.