Hi skabg. Welcome to the forums!

Does your blog work on the public side, meaning can you see it even though you can't get to the admin section?

I would start by assuming a file got jacked up somewhere somehow. Download the .zip package again, unzip it on your computer, DELETE the /skins_adm/ folder from your server, then upload the NEW /skins_adm/ folder to your server. If the problem was a corrupted file in that admin skin it'll be gone. If it wasn't then you'll know that's not the solution :)

Yes, the blog seems to work on the public side. I can post comments and view the demo articles, but I can't go in the admin section. It appears on the login screeen, that I'm logged in as an admin
"Note: You are already logged in as admin! Continue »", but when I click continue that error is what i see.
I have tried to remove all files and database and then reinstall the b2evo, but I get the same thing. Is it possible that the problem is with the skin? Is there way to change the default skin w/o entering the forum as an admin? maybe somewhere in the config files?

This is admin.php - Lines 161 to 167

161 * Load the AdminUI class for the skin.
162 */
163 require_once $adminskins_path.$admin_skin.'/_adminUI.class.php';
164 /**
165 * This is the Admin UI object which handles the UI for the backoffice.
166 *
167 * @global AdminUI

Problem solved. the blog was downloaded and unzipped thru SSH shell, then installed. I guess during the unzip process some files were corrupted or had different chmods.
Now i used FileZilla to re-upload the skins folder and everything seems to work.
Thank you for your help.

Congratulations, and thanks for reporting what you found and how you fixed it!