Guest sees page for 2 seconds then access denied to Group SP site

[David Slight]

So you may have seen my posts over the months as I try to use GROUPS to support external users (my customers). It has now been over a week on my latest support ticket with no response or even attempt at diagnosing the issues our tenant is facing. HUNDREDS of my external users are unable to access content (and yes they are all set correctly in AzureAD). All the permissions have been checked with PS. Why am I mad? Well this morning, I get an email from my support "ambassador" recommending I read a THIRD PARTY website on SharePoint permissions. It may be a good article but is that the best support can do? And anyway, it is the (expletive deleted) auto-provisioned site for a GROUP, I had nothing todo with how it is setup and the users permissioned. I just add someone to a GROUP and it all should work, well it did until abot two weeks ago. So I have tried the support route and it is not working so can someone tell me what these errors in the browser console log indicate? blah, blah ... 18. Ignored setting non-registered setting 'searchux.strings.RefinerPane.ariaRemoveRefinerLabel'. 12.sp-pages-search_a6bd02ab633026ef31aa.js (1,8979) 19. 2 HTTP400: BAD REQUEST - The request could not be processed by the server due to invalid syntax. (XHR)GET - https://iasahome.sharepoint.com/sites/cita-p003/_api/SP.Directory.DirectorySession/Group('13cb6371-f... 20. HTTP403: FORBIDDEN - The server understood the request, but is refusing to fulfill it. (XHR)GET - https://iasahome.sharepoint.com/_vti_bin/DelveApi.ashx/authtoken/loki?d=1521330891068 21. HTML1300: Navigation occurred. AccessDenied.aspx (1,1) 22. HTTP400: BAD REQUEST - The request could not be processed by the server due to invalid syntax. (XHR)GET - https://iasahome.sharepoint.com/sites/cita-p003/_api/SP.Directory.DirectorySession/Group('13cb6371-f... 23. HTML1506: Unexpected token. NB: I see the article that mentions this but after a quick review it doesn;t tell me how to fix the situation https://techcommunity.microsoft.com/t5/SharePoint-Support-Blog/Unexpected-Access-Denied-Invitations-...

[#2]

Did you get anywhere with this.  Sounds like the exact issue I have been experiencing for months.

Thanks

[#3]

If I recall correctly, in our case it was because there was a SharePoint level domain whitelist applied to the site, and the user had changed domain - so their old UPN was included in the whitelist, but their new UPN was not. We simply removed the whitelist in this case, and the issue went away.