I upgraded my site to Joomla 3.4 from 2.5. Before the upgrade I uninstalled the jdownload component as I have taken down that portion of my site and no longer need the extension. However, upon upgrade I get this error message:


Fatal error: Call to undefined method Joomla\Registry\Registry::loadJSON() in /xxx/xxxx/public_html/kingsofkustoms.com/plugins/system/jdownloads/jdownloads.php on line 38

I am no longer able to access the administrative backend of Joomla.

Hi,
when you have access to phpmyadmin, you should do this:
- open phpmyadmin with your joomla databse
- open the _extension table
- edit the item for the system plugin (see pics)
afterwards should you have access again to the backend.

Seems that you had let installed the old 2.5 system plugin from jDownloads.

Edit:
Okay when you need not more jDownloads, you should uninstall ALL jDownloads extensions. Please search in the extensions/manage page to 'downloads'. Then should you get listed all.

[gelöscht durch Administrator]

I followed the directions but now I get a new error:

Fatal error: Call to a member function set() on a non-object in /xxxx/xxxx/public_html/kingsofkustoms.com/administrator/components/com_login/controller.php on line 36

Are you able to help me further?

I think that the migration from Joomla 2.5 to 3.4 was not really succesful.
So you should restore a backup and try it again.
But you should read the update instructions for the migration process before you start it.
Please note that your problem has nothing to do with jDownloads. So please use a pure Joomla support forum for it (like Joomla.org).

I failed to mention that I still do not have access to the backend. And, no I do not think the migration completed. This is my fourth site I have migrated, following the Joomla migration procedures. I did not have any problems with any of the other sites. I will post my problem on the Joomla forums.

I failed to mention that I still do not have access to the backend.

This is clear since the Joomla login script has created this error:

Fatal error: Call to a member function set() on a non-object in /xxxx/xxxx/public_html/kingsofkustoms.com/administrator/components/com_login/controller.php on line 36

But as you can see that this is not a jDownloads issue.

I will post my problem on the Joomla forums.

Yes, this should be the best. I have not so much experiences with the Joomla migration script from 2.5 to 3.x.