iCloud Photo Library Link broken, can't reestablish link

Hoping to find some help with this one, as it is literally effecting my most important file type, Photos.

For the past few months (right after 10.10.3 dropped), every time I rebooted my iMac, I would be presented with a "sign in to iCloud" dialogue box, twice each time. If I entered a password, it would keep popping up. I finally cancelled it (it would go away after the second time) and found that everything was working. So it was a small annoyance. Suggestions I found online were to turn off iCloud and then turn it back on. I was hesitant to do so, because I have a 200 GB iCloud Drive, with over 100 GB of Photos stored there, and I didn't want any problems. I store full resolution on this iMac, and want to keep it that way (This is the machine that Time Machine backs up from)

So I figured I would wait until 10.10.4 to drop and see if that helped fix the issue. It did not. So this morning I turned of iCloud completely. Deleted Mail, Contacts, etc from my iMac, but it, obviously, left my Photos Library alone.

Sign back in to iCloud, reboot, and the problem is gone!

Great, right?

Wrong.

When I try to turn on iCloud Photo Library, again keeping full resolution on this iMac, I get the following error.

I can't have that, because I would then not have my full-resolution anywhere in my control, and not only that, but I would end up with 2 of every picture I have, all 100 GB, and that would put me at the max space of my iCloud Drive.

Anyone have any tips? This seems like it should be a simple thing, but I can't find any way to fix it.

Any and all help will be appreciated.