When I upgraded a site collection to version 2013 recently, I notice all the pages with custom page layout cannot be edited anymore. We did a test run on our local SharePoint site with the same content but didn’t run into this problem.
One thing pretty funny is, all SharePointers will know that SharePoint 2013 assemblies are version 15. But I noticed the version of custom page layouts on my SharePoint Online site are now version 126.96.36.199. I checked the version of my SharePoint Online site by navigating to https://myspsite.sharepoint.com/_vti_pvt/service.cnf and it shows also version 188.8.131.529
We have not find the solution to this, so wait for further posts on how to fix this.
Update: 1 day later, the site where the pages cannot be updated works perfectly without me doing anything. Could it because SharePoint Online migration actually will take a longer time beyond the ‘OK’ status.