This is nearly always a browser setting rather than a fault with the portal.
If it keeps happening
Your browser may be blocking third‑party cookies for embedded content. Allow them for this site, or try a different browser.
The deployment may have been replaced. Whoever manages the portal should check the deployment id in this page against Apps Script → Deploy → Manage deployments.
doGet() must keep setXFrameOptionsMode(ALLOWALL), or embedding is refused.