Access Denied - Sharepoint URL as Custom Tile in O365

Question

I am adding a custom tile as explained here with SharePoint page as URL. The issue I am facing is once we log in to O365 home page and then try to goto SharePoint page directly using custom tile we are getting Access denied message. And after that none of the SharePoint URLs work. All the permission for users are set appropriately. In fact, I have global administrator rights for O365.

The scenario that is failing:

1. User logs in to O365
2. Goes to App Launcher -> clicks custom tile
3. Gets Access denied message "Access Denied. You don't have permissions to access this resource. Sign in with the account provided by your work or school to use with Office 365 or other Microsoft services."

The scenario that works:

1. User logs in to O365
2. Clicks on SharePoint icon on the home page and goes to SharePoint home page
3. Goes to App launcher -> clicks on Custom tile
4. The page loads fine

Please let me know if there is any workaround or solution for this issue

Answer 1

The issue turned out to be the URL I used for "Image URL" was creating the issue. I copied the image url from SharePoint SiteAssets folder which had permission as "Any with the link can view".

So when we login to O365, SharePoint assigned guest access to the user for loading icon in App Launcher. Then when we visit to SharePoint URL using the App Launcher it continued to maintain the Guest Access.

After changing the Icon to some generic Icon the issue is resolved.

Though the issue is gone now, I am still not clear on how it did work in this scenario.

The scenario that works:

1. User logs in to O365
2. Clicks on SharePoint icon on the home page and goes to SharePoint home page
3. Goes to App launcher -> clicks on Custom tile. The page loads fine