I've been preparing an upgrade from SharePoint 2013 to 2019. So far I've managed to attach the database to 2016 and successfully upgraded it.
I was able to use the site collection without any problems. So I dismounted the database from 2016 and mounted it to 2019. Upgrade steps where successfully executed and I even ran psconfig (Gui) just to be sure.
My problem is that I get an access denied message when I try to open the site with farm/site collection administrator. Removing the site collection administrator and adding him again didn't help.
The access denied url contains a list guid which (using Powershell which works just fine) I traced back to the master page gallery. It seems I lost access to my master page gallery, which doesn't make a lot of sense to me. Can anyone help?
Update: Not sure if it's relevant, but it's a host named site collection.