How to gain entry for Admin after upgrade to 7.2.7?

Our ProjeQtOr was upgrdade from 7.1.5 to 7.2.7.
As usual, it shows a red message box stating

Upgrade in process.
Only Admin user can connect.

It requires the Admin (me) to log in first.
But ProjeQtOr does not accept my user name and password (something went wrong).
And if Admin cannot log in, the new version cannot start!

Question is, what is the way to gain entry?

We have another system user who has the top privilege (same as Admin).
But after the upgrade, he cannot login using his name and password.

Thus we are in a catch 22 situation.

How to break this impasse?

Of course, but it is not working. Now what to do?

Well, Damian.sd actually told me something that I already know and do as a routine, for each and every upgrade.
In that context I said that "it is not working".
Really, there is nothing else to report.

Let me rephrase the description:

After every upgrade, I see the same message "upgrade in progress/Only Admin user can connect".
As a routine, I do that and carry on with the latest version.

But not this time: It does not let me log in. It reports wrong login parameters.

So, my question was, if the admin cannot log in, what do I do next?
For the time being, we reverted to the old version.