Sorry this is occurring for your QC/ALM server.
Since you are on the QC Login page, that means that the APPLICATION is loading, at least, so it is not the QC application.
Since you say ALL users have the login issue, and the error says it is an "authentication" problem, it must be some issue with your LDAP authentication (if you were using QC-authentication, not LDAP, you would still have most users able to login and may have a few that "forgot" their password).
AS a test, you can revert to "QualityCenter" authentication:
You need to go into your database/schema called qcsiteadmin_db (default name for site admin schema) and set a parameter, as well as make yourself or a site admin user's QC-authentication password blank with these commands:
set QC to use "Quality Center Authentication":
update qcsiteadmin_db.td.params set PARAM_VALUE='QualityCenter' where PARAM_NAME='AUTHENTICATION'
Make sure your Site Admin User(s) have blank passwords to be able to get into Site Admin with a QC-auth password they know:
update qcsiteadmin_db.td.users set USER_PASSWORD='' where USER_NAME='QCADMIN'
(replace QCADMIN with your site admin user name)
Cycle ALM service and try logging in to Site Admin as "QCADMIN" (or your admin user) with a blank password.
Then, review the LDAP settings and test it's login to LDAP on the LDAP configuration page.
You can leave this instance of IE up and logged in to Site Admin, then set to use LDAP instead of "QC authentication".
Open a SEPARATE instance of IE and try logging in to Site Admin again, but this time with your Site admin user and their LDAP password. If it works, you have fixed it! (sometimes just testing and cycling out of LDAP to QC-auth and back resolves).
If not, then you need help from your LDAP/security people to help resolve LDAP settings.