We have the same problem too. Lots of issues over the last two days with various widgets not working, s123 widget is now back working but now the embeds not working, seems to be not liking the url domain its getting changed to https://undefined.undefined/..... We have a workaround with using iframes (code option) but this only works if you dont want to use "connect to data" and pass paramaters in the embed site url.
It appears to be somewhat fixed, works ok in firefox but in Edge or Chrome for some embeds we are recieving errors like "lmcc.maps.arcgis.com refused to connect" or "arcgis.com refused to connect" displayed on the page. The console shows errors: Refused to display 'https://lmcc.maps.arcgis.com/sharing/oauth2/authorize?client_id=dashboards&response_type=token&state=%7B%22portalUrl%22%3A%22ht...............%2Flmcc.maps.arcgis.com%2Fapps%2Fdashboards%2Fc76e3cb482214f359563dbb0e1ad07cb' in a frame because it set 'X-Frame-Options' to 'sameorigin'.
Note that these we working fine before the weekend on any browser/device. I am not aware of any software updates.
I have given you access to an experience builder - you can view it here Experience
Sometimes the embedded survey123 forms load other times they give the refused to connect message
Select a record on the list for the connect to data embed to work. The refused to connect always appears in the bottom left which is a dashboard passing a parameter (objectid).