I turned off all plugins, including domain mapping, and I still couldn’t log in. I looked through the code in my wp-config.php file and there was nothing related to cookies.
I have a number of plugins running now on my network, but I did try deactivating them all and then logging in to the umtentrepreneur.com site and I got the same error message. This is my first time setting up the new website/domain. I never tried logging in before mapping it, only after.