Hi,
I am trying to sign into OWA 2013 with a 2010 mailbox user and I just get the following page
:-(
something went wrong
Sorry, we can't get that information right now. Please try again later. If the problem continues, contact your helpdesk.
Want to be able to use your email when you're offline?
Next time you are connected:
1. At the top of Outlook Web App, next to your name, click Settings 
2. Click Offline settings and turn on offline access
3. That's it! You'll never see this message again.
I checked the logs and it looks like it might be trying to redirect to the Exchange 2010 Front end.
Exchangecas13 is the 2013 CAS server, Exchangefe1 is the 2010 CAS/Transport Server.
2013-07-26T20:42:01.691Z,8e66a7cb-63bc-4651-b769-d0062f7c7665,15,0,712,12,,,,,,,,,,,,,EXCHANGECAS13,,,,,,,,,,,,,,,,,,,,,,,,,,600050.0544,,,,,,,,,,,,,,,,,,S:ActivityStandardMetadata.Action=GlobalActivity;I32:ADR.C[SUPPR]=6;F:ADR.AL[SUPPR]=2.428583;I32:ADS.C[SUPPR]=1;F:ADS.AL[SUPPR]=2.1359,2013-07-26T20:42:18.433Z,17bed7ea-9bce-4759-bdd5-c8a25667f521,15,0,712,12,,Owa,exchangecas13,/owa/,,FBA,False,,,,Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.1; WOW64; Trident/6.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; .NET4.0C; .NET4.0E; MS-RTC EA 2; Media Center PC 6.0; InfoPath.3),192.168.1.30,EXCHANGECAS13,302,,,,,,,,,,,,0,,,,,,,,,,,,,2,,,,,,,,,,,,1,1,1,1,,,OnBeginRequest=0;NoCookies=302 - GET/E14AuthPost;,
2013-07-26T20:42:18.438Z,41574515-5ea0-43b4-8a13-d327ec109d19,15,0,712,12,,Owa,exchangecas13,/owa/auth/logon.aspx,,FBA,False,,,,Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.1; WOW64; Trident/6.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; .NET4.0C; .NET4.0E; MS-RTC EA 2; Media Center PC 6.0; InfoPath.3),192.168.1.30,EXCHANGECAS13,200,,,,,,,,,,,,0,,,,0,,,,,,,,,1.9969,,,,,,,,,,,,1,1,1,1,?url=https%3a%2f%2fexchangecas13%2f&reason=0,,OnBeginRequest=0;,
2013-07-26T20:42:18.464Z,02c9d811-e43d-4dab-a1eb-78a85b73f3e5,15,0,712,12,,Owa,exchangecas13,/owa/auth/logon.aspx,,FBA,False,,,,Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.1; WOW64; Trident/6.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; .NET4.0C; .NET4.0E; MS-RTC EA 2; Media Center PC 6.0; InfoPath.3),192.168.1.30,EXCHANGECAS13,200,,,,,,,,,,,,0,,,,0,,,,,,,,,2.0001,,,,,,,,,,,,1,1,1,1,?replaceCurrent=1&url=https%3a%2f%2fexchangecas13%2f,,OnBeginRequest=0;,
2013-07-26T20:42:24.930Z,54e8f566-4c32-4fbe-9e84-d56bee916785,15,0,712,12,,Owa,exchangecas13,/owa/auth.owa,,FBA,True,LWGINC\pslager,,Sid~S-1-5-21-1186999560-1424097373-3473557-14162,Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.1; WOW64; Trident/6.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; .NET4.0C; .NET4.0E; MS-RTC EA 2; Media Center PC 6.0; InfoPath.3),192.168.1.30,EXCHANGECAS13,302,,,POST,,,,,WindowsIdentity,,,,121,,,,1,17,,0,,0,,0,0,19.9095,0,,,,,,,,17,1,,18,18,19,19,,,OnBeginRequest=0;DownLevelTargetHash=0/0/1;ClientAccessServer=EXCHANGEFE1.LWGINC.com;ResolveCasLatency=0;,
2013-07-26T20:42:24.937Z,f81832cf-43c4-49ed-942f-3ff1e951bd6a,15,0,712,12,,Owa,exchangecas13,/owa/auth.owa,,FBA,True,LWGINC\pslager,,Sid~S-1-5-21-1186999560-1424097373-3473557-14162,Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.1; WOW64; Trident/6.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; .NET4.0C; .NET4.0E; MS-RTC EA 2; Media Center PC 6.0; InfoPath.3),192.168.1.30,EXCHANGECAS13,302,,,POST,,,,,WindowsIdentity,,,,121,,,,0,1,,0,,0,,0,0,2.9963,1,,,,,,,,1,0,,2,2,2,2,,,OnBeginRequest=0;DownLevelTargetHash=0/0/1;ClientAccessServer=EXCHANGEFE1.LWGINC.com;ResolveCasLatency=0;,
Any ideas?