I recently came across an issue where SharePoint online sub became inaccessible during a tenant-to-tenant migration. There was a site synchronization happening at the time. Whenever I opened one of the sites being migrated, I received an error saying sorry something went wrong. This wasn’t a page not found error so I knew it had to be related to the site config.
I began to investigate by looking at the site contents, features and settings. I could confirm that the site contents were still present however I could not open any list or document library on the site. Even worse while I could view the site settings page I could not open any of the site settings options. After some searching online, I found there was not much information on this issue for SharePoint online.
I went through many potential causes that I had found online but could not get into any of the settings pages to confirm any of the issues. In the end the issue turned out to be quite easy to fix once you know where to look. During my troubleshooting, one of the issues that I found online was related to the site master page. If the master page setting is configured incorrectly then this can lead to issues very similar to what I’ve seen.
With the site settings unavailable, I could not use the navigation to check the master page settings. This is where PowerShell comes in handy. Using the PNP online PowerShell module I connected to the specific subsites that I was having issues with and checked the existing settings for the master page of the sites.
What they found is that the master page had been changed from the standard Seattle dot master file to a custom master file in the source environment. When the sites finished synchronizing this was updated in the destination however the custom master file was missing.
To regain access to the site I simply reset the master file setting back to the default Seattle.master URL for each site that was experiencing issues. This allowed me to regain access to the settings and view the content of the site.
I’m not very experienced in SharePoint customization or development so even though I work quite a lot with SharePoint Online this was particularly hard to troubleshoot. I decided I would upload this issue here to help any other admins who experience similar issues.