Can't log in to either of the two Power CMS installations

I have purchased two copies of Power CMS. They were correctly and successfully installed in two different domains. I haven't logged into either of them for months.

I had to make some changes to one of the sites through Power CMS, so i tried to log into one of them. I used the correct username and password, however, the login failed, and it just returned to the blank login page.

The database checks out as good, and the username and password are correct.

I noticed that there was an updated version available, so i reinstalled Power CMS.
It installed without a problem and, during the installation, I set the username and password.

I logged out, then tried to log in again. The same thing happened. I could not log in, and it reset to the blank login page.

I decided to check the other domain where I have my second copy of Power CMS installed. The same thing happened there. The login reset to the blank login page.

I called their hosts technical support, and spent a considerable amount of time on the phone with them. They checked through everything on their end, and everything checked out as good. When they tried to log in, they had the same experience as I did.

So there are no problems on the hosting side, so logic sent me here.

What is causing the login problem??????

A quick response would be appreciated as my client is waiting for me to enter a time-sensitive promotion.

thanks.