I found a post from someone having the same exact problem. The answer didn't help.

I had a power problem recently,  and when fixed,  my network was showing 'Motorola' as a network name. I tried connecting my tablets to it and they connected no pronlem. But then I noticed under security it said 'none'. I powered it still the same. Finially, did a hard reset. Got back original name. Got  tablets, and phone online. Then tried to access the web manager. Logged in using admin & Motorola. Took me to 'change password' page I entered admin, and motorola, and new password

'Oops! Invalid current user/password'. 

I've tried motorola,  the wifi key, with and without spaces.

I've tried another hard reset. Power reset, and finally, ANOTHER hard reset.

I manage to get to the change password page no problem. But no matter what I enter for user name and password, I get 'Oops! Invalid current user/password'

Pulling my hair out here.

I broke down and ordered a SBG8300 today. Since Xfinity upgraded my service and raised my fees $5.00 a month, (without asking if I wanted it,) I've spent at least 8-10 minutes a day watching arrows chasing themselfs. We'll see if docsis 3.1 fixes that. But in the meantime I still need to access the web manager on this SBG6580. Any help would be appreciated.