I tried to reproduce the issue (using Joomla Updater) but I get a different error:

on URL http://localhost/joomla2.5/administrator/index.php?option=com_joomlaupdate&task=update.finalise

It shows that error both in front- and backend.

Disabling the admintools plugin in the database fixes the frontend but brings then another fatal error in the backend:

Probably because the update failed during the finalise step.

So either the test instructions to reproduce this issue are wrong or I do something wrong.

But then this issue isn't really a Joomla issue anyway. It's caused by the user who didn't make sure the installed extensions are compatible with Joomla 3.2 prior to the update. This is a mandatory step when going between major versions and there is no excuse for not checking that prior to hitting that funny update button.

The error message you got has to do with that old version of Admin Tools not being compatible with Joomla! 3. I forgot to say that you need to disable that plugin before upgrade. Sorry!

The fatal error in the backend comes from the post-installation script not running because of the error you got when the update.finalise task was called and never run. The correct test instructions are:

• Create a new Joomla! 2.5 site
• Install Admin Tools Core 2.2.10
• Uninstall Admin Tools Core. This leaves behind the old version of FOF in libraries/fof
• Upgrade to Joomla! 3.2
• You will get one of the aforementioned error messages

Still don't get the error you see:

This may be due to different server settings.
Removing the old FoF files manually solves that issue. Installing the current version of AdminTools solves this issue as well (as it apparently also does the cleanup).

On a sidenote: I was surprised that FoF didn't show up in the extension manager. If it did, users could uninstall it properly prior to the update. Later versions show up like expected. So I guess this is an issue with FoF v1.

So basically we have an issue with a very old version of an external framework which can't be uninstalled properly. And we have to fix it because we now have a properly coded successor version of FoF in core. Sounds a bit wrong but I guess since it's easy to do and helps the user we can do it anyway.

Another approach as I understand it would be to change the FoF autoloader so it doesn't load the legacy classes first in Joomla 3.x. Since those classes aren't compatible with Joomla 3.x anyway it doesn't make much sense to try loading them at all. I guess it's needed for B/C reasons in Joomla 2.5?

Can you update the test instructions in the tracker and in the initial post here as well? Otherwise testers will have a hard time reproduce the issue.

As I said, the error messages I pasted above are some of the common ones. Depending on what is running on your site, in which order, etc it may cause different error messages. If it has to do with com_postinstall or an FOF class name it's this issue.

FOF 1.x wasn't a library package on purpose. Back then we had to support Joomla! 1.5 and 1.7/2.5 in a single package. The only way to do that was to shovel files in the libraries directory.

Changing the FOF autoloader would break b/c. I'm not sure if anyone uses that feature, but I don't want to find out by braking b/c in a sub-minor version upgrade. I try to follow semantic versioning with FOF. I will change the autoloader for FOF 3 but not anytime sooner. I'll update the instructions right now.

Thanks for updating the instructions.

@test
It did not delete the said files here. I started from a 2.5.16 and used the extension manager Install with an update pack containing the patched script
I get
Fatal error: Class FOFController cannot extend from interface JController in /Applications/MAMP/htdocs/Joomla_2.5.14-Stable-Full_Package/libraries/fof/controller.php on line 23

and indeed the files are still present in the library

Hi,

I upgraded a site and am now getting this error in the admin backend:
Fatal error: Class FOFModel cannot extend from interface JModel in /home//public_html/libraries/fof/model.php on line 21

What do I do to fix this?

You have tried to install a very old FOF-based component which was designed for Joomla! 1.5 only. This screwed up your installation. You should be able to easily fix it by doing the following steps in the order specified:

• Delete the libraries/fof directory from your site
• Download FOF 2.1.1
• Extract the ZIP file locally
• Upload the fof directory you extracted into your site's libraries directory Assuming that the older component you installed isn't using a private copy of FOF 1.x and loading it via a plugin you should get access to your site. Otherwise you will have to determine which plugin is trying to load the very old FOF version and delete it from your site.

Hi Nicholas,
  Thank you very much for your help! This has gotten the backend of
  the site back for us. I had a forum post about the problem, so
  have added your fix for anyone else that is having the issue.
  As far as I know we have all Joomla 2.5 and 3.x extensions
  installed. Is there any easy way to figure out what extension used
  the old FOF library?
  Thank you again so much for your help, it is really appreciated.
    Laurelle 

You could check that all your 3rd party extensions are up to date ?

Thank you Nicholas. Replacing FOF libraries fixed the upgrade issue. Got my control panel back.
_Cindy

Hi,

I'm also experiencing this problem with the following error "Class FOFModel cannot extend from interface JModel in E:\wamp\www\site1\libraries\fof\model.php on line 21"

In my case this error only appears in ""administrator/index.php", if I type "administrator/index.php?option=com_installer&view=manage" in browsers address bar, I can use administrator panel without problems, but every time I access "administrator/index.php'' this error appears.

After updating from joomla 2.5.17 to Joomla 3.2.1, I checked fof version, it was 2.1.1. But I tried above mentioned fix anyway, sadly it didn't help me. I also tried deleting fof directory entirely, but that brought a whole new set of errors.

I didn't have any old components or plugins, in fact, I deleted everything not compatible with Joomla 3 and updated everything that is compatible with Joomla3 and I didn't event have that much components/plugins.

I have a backup so I could try to update again, but could you please suggest what to do in order to succeed? If I would update fof to 2.1.1 before updating Joomla to 3.2.1 would it help to avoid this error?
I also tried disabling every plugin and disabling every unprotected component, sadly this also didn't help.

Thank you nikosdion - replaced fof and the backend blank page has gone.

I didn't delete libraries/fof folder before. I just replaced it. I tried deleting it first and the replacing it, that helped.

Thank you!

Nikosdion,

Thanks very much. The challenge with using Akeea across many hosts is that I am not quite sure what I will get out it. Still better than manual in most cases.

Steve