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

Finally found a fix!
This seems to be the bit of code that should be running it in administrator/components/com_k2/views/item/tmpl/default.php, around line 720

I have added <?php JHTML::_('behavior.modal'); ?> which has fixed it, but not sure if this is correct PHP and if this then goes on to break anything else?


<?php if($this->mainframe->isAdmin() || ($this->mainframe->isSite() && $this->permissions->get('editAll'))): ?>


<?php JHTML::_('behavior.modal'); ?>

<a class="modal" rel="{handler:'iframe', size: {x: 800, y: 460}}" href="index.php?option=com_k2&amp;view=users&amp;task=element&amp;tmpl=component"><?php echo JText::_('K2_CHANGE'); ?>
</a>

<input type="hidden" name="created_by" value="<?php echo $this->row->created_by; ?>" />

<?php endif; ?>

Confirmation of this fix. I was driving myself nuts trying to change the authors. K2 devs should look at this and fix. Don't wan to have to do this eveytime I install k2.....

Worked for me. Thanks for the fix.