Manual upgrade 9.1.2 to 9.1.5: "Ein Fehler ist aufgetreten"

I did a manual upgrade (removed old oc files, added new ones).
I opened the web site in my browser (Firefox) and started the upgrade like requested by the site.

Aktualisierung auf 9.1.5

Ein Fehler ist aufgetreten.
Detaillierte Fehlermeldungen
Ein Fehler ist aufgetreten.
Bitte laden Sie die Seite neu.

After reloading the page it seems that the upgrade worked, current version is 9.1.5.
What does the error message above mean?

Most likely some caching issue within your browser causing this. If the version is now at 9.1.5 everything went fine and i think you shouldn't be worry about this.

If you want to be absolutely sure you can check your data/owncloud.log to see if there are any reasons for this message logged there.