@Sven Magnus Hi Sven. I'm using Supabase with Sveltekit too and I have no problems signing in. Below is my script from the login.svelte route page.

<script>
  import { supabase } from '$lib/supabaseClient'
  import { goto } from '$app/navigation'
  import { user } from '$lib/stores/sessionStore.js'
  
  let loading = false
  let email = ''
  let password = ''

  const handleLogin = async () => {
    try {
      loading = true
      const { error } = await supabase.auth.signIn({ email, password })
      if (error) throw error
      const tmpUser = supabase.auth.user()
      user.set({
        email: tmpUser.email,
        id: tmpUser.id,
      })
      goto('/')
    } catch (error) {
      alert(error.error_description || error.message)
    } finally {
      loading = false
    }
  }
</script>