OK but it is a proper practice to have this redirect. Without it, sometimes Google will see
www.domain.com/index.php
and
www.domain.com/
as being a duplicate of the same content, this hurts rankings. Probably  most people don't know this but proper SEO calls for this type of solution. Can your system be set up to work with canonical redirects?

OK this keeps getting more confusing.

Before I logged out, I changed the login/pw to some dummy text and tried to run it again. Not login or password error, actually now I get the same exact error message I had before, about the home page can't be guessed. Seriously?

So maybe there was a login/pw problem all along and nothing to do the the URL issue. Not very impressed so far.

First, there is an error message that says the system can't guess my home page, to enter it at the end of the URL. So I added /index.php to the URL, producing:  www.domain.com/index.php. That doesn't work either -  a box pops up and says to enter it *exactly* as it appears in a browser which is what I did before without success. This seems contradictory and  makes no sense because the browser loads www.domain.com/ just fine.  In fact,  that's the only way it will load, since a properly set up site will not allow both www.domain.com/ and www.domain.com/index.php to load. So as a matter of proper practice there is a 301 redirect from /index.php pointing back to www.domain.com/ to prevent a duplicate URL from occurring, not sure how this should be causing any problem. I sent in an email about this, did not receive an answer after 2 hours, so hoping to get a faster answer here.