What you are describing — unclickable Sign In button and non-functional tab switching is a could becknown rendering issue in the modern Dataverse form editor tied to z-index/iframe layering in Chromium-based browsers. Here is what to try:
1. CLEAR BROWSER CACHE AND COOKIES
Even in incognito mode, some browser extensions or cached service workers can interfere. Try a full cache clear in Edge/Chrome, then reload the maker portal.
2. DISABLE BROWSER EXTENSIONS
Ad blockers, password managers (like LastPass or 1Password), and privacy extensions are known to overlay invisible elements that block clicks. Try in a browser profile with zero extensions.
3. CHECK FOR TENANT CONDITIONAL ACCESS AFFECTING THE SIGN-IN PROMPT
The Sign In prompt inside the form editor usually appears when the editor iframe loses its authenticated session (often triggered by Conditional Access policies that scope session tokens to the main browser context but not iframes). Your admin can check if there are CA policies applying continuous access evaluation to maker portal iframes.
4. SWITCH ENVIRONMENTS
Sometimes the issue is environment-specific (solution layers or form metadata). Try opening the same form in a different environment to confirm whether it is environment-wide or browser-specific.
5. USE CLASSIC EDITOR AS TEMPORARY WORKAROUND
You have already found this — keep using classic editor for now while you raise a support ticket.
6. RAISE A MICROSOFT SUPPORT TICKET
Since this reproduces across Edge and Chrome in incognito mode, it is likely a product bug. File a support ticket via the Power Platform Admin Center with a browser HAR file captured during the failure — this gives Microsoft the iframe request trace they need to diagnose it.
