Why do I get the error "should not start with "$" or "_" which are reserved prefixes for Vue internals."?

Question

I am getting the following error when I am in my Login component:

[Vue warn]: setup() return property "$subscribe" should not start with "$" or "_" which are reserved prefixes for Vue internals. 
  at <Login backend="http://localhost:1337" onVnodeUnmounted=fn<onVnodeUnmounted> ref=Ref< undefined > > 
  at <RouterView backend="http://localhost:1337" > 
  at <App>

I get multiple similar errors from other variables in runtime-core.esm-bundler.js.

Everything was working previously and I believe no changes were made to the code. I think it is coming from my setup() in the login page which returns Pinia:

    setup() { 
      const userStore = useUserStore()

      return userStore
    },

I am not sure how to solve this issue, since the error seems to come from a bundle!

EDIT: I have edited my setup() as per Estus Flask's suggestion. I now receive an error when trying to pass the credentials of the user to the server.

[Vue warn]: Unhandled error during execution of native event handler 
  at <Login backend="http://localhost:1337" onVnodeUnmounted=fn<onVnodeUnmounted> ref=Ref< 
Proxy { <target>: {…}, <handler>: {…} }
 > > 
  at <RouterView backend="http://localhost:1337" > 
  at <App>

When I use hardcoded credentials it works, but when I try to use the same credentials through the form the error appears.

Here is the form:

<form @submit="login" class="login-form">
        <label class="input-label">User Name</label>
        <input type="text" required class="input" v-model="credentials.username" @input="updateCredentials" />
  
        <label class="input-label">Password</label>
        <input type="password" required class="input" v-model="credentials.password" @input="updateCredentials" />
  
        <input type="submit" value="Log in" class="button loginButton" @click="login"/>
        <input type="button" value="Register" class="button registerButton" @click="register" />
      </form>