I just recently set up my 2013/2010 coexistence network with 1 EX2010 server running all roles and 1 EX2013 running both roles. There is a TMG firewall at the external front of the network which shouldn't really come into play here.
When I first set up the 2013 CAS, it pulled in all the EX2010 info. When I would log in to localhost/owa on the 2013 CAS as a 2010 user, I would get proxied no problem. I was in the process of pointing all DNS records and firewall ports to the new 2013CAS when I noticed that now when I login to the 2013CAS, even just locally, using a 2010 mailbox, it starts to proxy and then gets stuck "Still Working On It". Every time I try and log in with a 2010 user it logs in fine and then gets stuck and the circle keeps spinning. It seems to make it to the 2010 OWA because if I stop the page, the favicon for the old 2010 OWA is shown at in the address bar.
I am able to login to the EX2013 box locally with a 2013 test user and it pulls up the 2013 OWA. I can also log in to the 2010 OWA on the EX2010 server no problem. I have checked both OWA pages and they are set to FBA.
I have tried changing all the authentication methods, changing the external URLs on the 2010 box from the same as 2013, to null, to legacy and nothing worked. I Have recreated the 2013 virtual directory but haven't recreated the 2010 virtual directory because I wanted to make that a last resort option just in case. I have also tried looking into the IIS logs but am not too sure how to read through them.
Hopefully someone has some things that I can try or some logs that I can check to see if I can find where the issue is. If you need any more information or want me to provide some logs I would be more than happy to provide.
Thanks in advance for any help.
Tmark86