This has been fixed in the RC. Now works perfectly fine.

Just performed the upgrade from beta to RC. Went very smooth with yum upgrade.
All works well, no changes were made to the altered configurations, changes I made to NFS, exports, firewall, containers and passing thru of USB controllers still intact. Very smooth upgrade.
New version of XOA also now handles userid and password correctly.

Good job!

and yes I can log in using admin@admin.net with admin as the password, but not with the credentials I can set when installing.

Hmm, maybe to clarify, I didn't install from https://xen-orchestra.com/#!/xoa, I installed from by going to the ip address of my xcp-ng host, and installed from there. Using the 8.0.0 release

installing the appliance works like the dream, so good stuff there, will let you know if I find sth.

Spot on!
that indeed lets me log in

I tried to setup a new XOA this morning, directly going to the ip address of the host, and getting the nice UI to install the XOA virtual appliance. I went through the simple install, added all the details, including my userid and password, and the appliance got installed nicely. But, I can't login with the credentials I supplied. I tried it 3 times, each time it didn't work.
Is there anything I could be doing wrong? I didn't register my XOA, but according to the screen I don't have to. I also didn't use a username with a XXXX@YYY.ZZ contruct, but just a simple username.