diff --git a/src/features/party/PartiesProvider.tsx b/src/features/party/PartiesProvider.tsx
index 36a2d085f9..1338f65cbf 100644
--- a/src/features/party/PartiesProvider.tsx
+++ b/src/features/party/PartiesProvider.tsx
@@ -9,7 +9,7 @@ import { delayedContext } from 'src/core/contexts/delayedContext';
 import { createQueryContext } from 'src/core/contexts/queryContext';
 import { DisplayError } from 'src/core/errorHandling/DisplayError';
 import { Loader } from 'src/core/loading/Loader';
-import { instanceQueries, useInstanceDataQueryArgs } from 'src/features/instance/InstanceContext';
+import { instanceQueries, useInstanceDataQueryArgs, useLaxInstanceId } from 'src/features/instance/InstanceContext';
 import { NoValidPartiesError } from 'src/features/instantiate/containers/NoValidPartiesError';
 import { flattenParties } from 'src/features/party/partyUtils';
 import { useShouldFetchProfile } from 'src/features/profile/ProfileProvider';
@@ -117,6 +117,7 @@ const { Provider: RealSelectedPartyProvider, useCtx: useSelectedPartyCtx } = cre
  */
 const SelectedPartyProvider = ({ children }: PropsWithChildren) => {
   const validParties = useValidParties();
+  const instanceId = useLaxInstanceId();
   const [sentToMutation, setSentToMutation] = useState<IParty | undefined>(undefined);
   const { mutateAsync, data: dataFromMutation, error: errorFromMutation } = useSetSelectedPartyMutation();
   const { data: partyFromQuery, isLoading, error: errorFromQuery } = useSelectedPartyQuery(true);
@@ -131,7 +132,7 @@ const SelectedPartyProvider = ({ children }: PropsWithChildren) => {
     return <DisplayError error={error} />;
   }
 
-  if (!validParties?.length) {
+  if (!instanceId && !validParties?.length) {
     return <NoValidPartiesError />;
   }
 
diff --git a/test/e2e/integration/signering-brukerstyrt/existing-instance-access.ts b/test/e2e/integration/signering-brukerstyrt/existing-instance-access.ts
new file mode 100644
index 0000000000..616424478b
--- /dev/null
+++ b/test/e2e/integration/signering-brukerstyrt/existing-instance-access.ts
@@ -0,0 +1,64 @@
+import { AppFrontend } from 'test/e2e/pageobjects/app-frontend';
+import { Tenor } from 'test/e2e/support/users';
+
+const appFrontend = new AppFrontend();
+
+describe('Existing instance access', () => {
+  it('allows access when the user has no party allowed to instantiate', () => {
+    let userIsReopeningInstance = false;
+
+    // Regression setup for https://github.com/Altinn/altinn-studio/issues/19576:
+    // PartyProvider used to require at least one party allowed to instantiate before rendering anything, including
+    // an existing instance. Consequently, a user with access to an existing instance saw NoValidPartiesError (403)
+    // solely because they could not create a new instance. The test user can normally instantiate this app, so
+    // both the buggy and fixed frontend would pass without this mock. Returning no allowed parties when reopening
+    // the instance recreates the failing condition and ensures the test detects that unconditional check if it is
+    // reintroduced. The user's access to the existing instance itself remains real and is verified below.
+    cy.intercept('GET', '**/api/v1/parties?allowedtoinstantiatefilter=true', (req) => {
+      const isReopeningInstance = userIsReopeningInstance;
+      if (isReopeningInstance) {
+        req.alias = 'partiesAllowedToInstantiate';
+      }
+      req.continue((res) => {
+        res.send(isReopeningInstance ? [] : res.body);
+      });
+    });
+
+    cy.startAppInstance(appFrontend.apps.signeringBrukerstyrt, {
+      cyUser: null,
+      tenorUser: Tenor.users.humanAndrefiolin,
+      authenticationLevel: '2',
+    });
+
+    cy.findByRole('heading', { name: 'Hvem vil du sende inn for?' }).should('be.visible');
+    cy.findByRole('button', {
+      name: new RegExp(`org\\.nr\\. ${Tenor.orgs.sivilisertAvansertIsbjoernSA.orgNr}`, 'i'),
+    }).click();
+
+    const companyName = 'Testselskap AS';
+    cy.findByRole('textbox', { name: /navn/i }).type(companyName);
+    cy.waitUntilSaved();
+
+    cy.url().then((instanceUrl) => {
+      const instanceHash = new URL(instanceUrl).hash;
+      const instanceId = instanceHash.match(/\d+\/[\da-f-]{36}/i)?.[0];
+      expect(instanceId, 'instance ID').to.exist;
+
+      cy.intercept('GET', `**/instances/${instanceId}`).as('existingInstance');
+      userIsReopeningInstance = true;
+      cy.startAppInstance(appFrontend.apps.signeringBrukerstyrt, {
+        cyUser: null,
+        tenorUser: Tenor.users.humanAndrefiolin,
+        authenticationLevel: '2',
+        urlSuffix: instanceHash,
+      });
+
+      // Access to an existing instance must depend on the instance request, not whether the user can instantiate
+      // the app. The latter is only relevant when entering the app without an instance.
+      cy.wait('@partiesAllowedToInstantiate').its('response.body').should('deep.equal', []);
+      cy.wait('@existingInstance').its('response.statusCode').should('eq', 200);
+      cy.findByRole('textbox', { name: /navn/i }).should('have.value', companyName);
+      cy.get(appFrontend.instanceErrorCode).should('not.exist');
+    });
+  });
+});