Update procedure can't delete old files

Can someone help me, please? With most updates since 25.x, and also this one from 26.0.0 to 26.0.1 the web upgrading routine always stops at “Delete Old files” - I always have to restore from backup manually, and try to run the updater again. Ultimately this works then, but it is a big hassle.

Deleting old files requires php to have full access so it can delete the files. Ideally in the logfiles it should show some permission errors. If the restore from backup works for you, I’d check how the permissions change from the current state vs. when you play back a backup.