Unable to login to Xenforo forum

Hello, our company has purchased XenForo a few days ago, and it has been working perfectly, until there some problems occured to our server (IIS had to be reinstalled and reconfigured, multiple restarts, etc).
Now for some reason users are no longer able to log in, from any groups. The threads are displayed fine, members are still registered (also the datatables seems to be OK).
What’s really strange, that when we type false password on login, there is a feedback that its incorrect. But when the password is good, we are not logged in, no error messages are displayed, just throws back to the original page (as guest). So it seems like its checking the user credentials, but its not creating the sessions/cookies.
We tried registering a new user, and it also worked fine: after the registration he was logged in automatically. We could post threads etc, but after it logged out, we couldnt re-login any more.
We already reinstalled XenForo multiple times, and tried to reset the database from backup, however the same problem occurs.
Could you please recommend any possible solution?