[xwiki-users] Severe problem, can't login as admin, neither as superadmin

Hello List, I just had installed tomcat/xwiki.war/xwiki.xar. I can access the site, connection to mysql is working and mysql get's changed. Now the problem: If i login as superadmin (is enabled) or as admin, i get the login again with NO error-message!! If i do the same, but take an non-existing user, or a wrong password, i get an error-message, so i can be pretty sure, the authentication is basically working. In the logs i get nothing more than: .. !remember-me cookie validation hash mismatch! !remember-me cookie has been tampered with! !remember-me cookie is being deleted! !remember-me cookie validation hash mismatch! !remember-me cookie has been tampered with! !remember-me cookie is being deleted! What does this mean? Unfortunately, i neither found a documentation explaining all the different and possibly unvisible "xwiki.authentication.xxxx"-variables (xwiki.cfg), nor do i find something in google on this. I tried "xwiki.authentication.cookiedomains= "(w/o value), but this didn't change anything... PLEASE help, i have no clue why this authentication is not working correctly.... :( Kind regards, Florian