Error at upgrading addOn

[Helmut2509]

I have three applications, which have the same addOn to be upgraded.
With two of the applications, the upgrade works fine.
But with the third application I always get the error:
"Please enter a valid callback method (AvatarGallery_Listener::load_class)."
On the page
http://xenforo.com/community/resources/add-on-install-upgrade.960/
it is simply recommended to repeat the upgrade, but that is of no avail.

 

[Chris D]

That usually indicates you have uploaded the files incorrectly.

Ask in the discussion thread for the add-ons that are failing to install for support.