Problem with login form! Please, help!

Hello!
I have a multisite. On my first site I use BuddyPress. On my second site I use Ultimate Member plugin which I have a little trouble with. UM plugin creates user accounts very well, but when a user logs out and then tries to login again, the plugin says “Sorry, we can not find a user with this login name” and denies to access. And when it does that, at the top of the page a note appears that says: “Notice: Undefined index: user_login in /www/bionomic/www/htdocs/wp-content/plugins/ultimate-member/core/um-actions-login.php on line 45”. I have tried to make many test-accounts and the plugin has done the same to all of them. I have no idea what it may be. Please, help!