I am seeing the following error text on my website when using the User plugin.
Warning: Cannot modify header information – headers already sent by (output started at /home/ABC/public_html/plugins/user/raidplanner/raidplanner.php:1) in /home/ABC/public_html/plugins/user/joomla/joomla.php on line 181
There was a space before the <? sign in plugins/user/raidplanner/raidplanner.php file.
If you remove it, it will work. Sorry for the inconvenience.
(Or you can reinstall the latest version from github, which have this space removed)