Session question

I have this setup:

domain.tld - project site;
server.domain.tld - Hestia server;

The problem:

domain.tld has user management system which requires login. When logged in, cookie for user X is set.

Now, I go to server.domain.tld to login to the Hestia control panel. I submit credentials and immediately redirected back to login screen.

Issue resolves when I clear *.domain.tld cookies from browser or use “private browsing”. So far, tested on Firefox. Is this something related to Hestia cookies?