Header plugin generate error after Joomla! login
It seems like this problem didn't happens on all sites, but only in some configurations. Directly after login user is redirected to the blank page with text:
{"err":""}
After refreshing page appear standard Joomla! control panel page and error didn't appear until next login attempt.
Can be tested here:
http://181.224.130.224/~sigsiu52/test2/administrator/index.php