Maybe test your findings with firebug as I did in a previous post, and record them here.
The developer did intervene in this post a while back, but seems to have lost interest.

With another users finding they may take notice?

I just wish I knew a way to easily downgrade to K2.4.1 or K2.5.7 mine was all working fine then.

how do you do that? i have firebug installed but dont know how to do this report?

Open your backend K2 and navigate to an item
Open Firebug & select console > Errors
Then click the change author link
Then Click on another auther to change if see if it shows any errors anywhere.

For me when I click on Change author, it no longer opens in a modal pop up window, but a new screen.
When I click on another author (in the new screen which should be a popup) I see an error as i reported a few weeks back.
This is the only place where I see an error

Attached it as a screenshot here too.

I got exactly the same error! see picture

http://imageshack.us/f/805/43925543.jpg/

K2 Support or DEV's please could you take a look at this. Mine is not an isolated indecent.

You can have access to my FTP and superadmin if you need to take a quick look

Thanks

Andty

ok that's very very weird as i have uninstalled and clean manually my database, uninstall all non original joomla plugin

I have reinstalled K2 and still having the same issue?

julien wrote: ok that's very very weird as i have uninstalled and clean manually my database, uninstall all non original joomla plugin

I have reinstalled K2 and still having the same issue?

In J1.5 Julien?

Nop Joomla 2.5.5 and K2.6.2

All my problems are in a J1.5 site, since upgrading to K2.6.x

Im not using joomla 1.5 but only 2.5