Exchange 2013 ECP unable to login

I've had a similar issue, and there are plenty of other suffers out there too. There's a bug in CU2 that can cause problems with the install of the ECP site, which is what bit me.

Assuming you have both the CAS and database server roles installed on this Exchange server then it may not be enough to simply delete and re-create the CAS ECP virtual directory without doing some tidying up inbetween.

To fully re-create your ECP site, you may need to carry out the following steps (as per this blog entry)

1. Delete the CAS ECP web application: Remove-EcpVirtualDirectory -Identity "Exchange01\ecp (Default Web Site)"

2. Remove MSExchangeECPAppPool from IIS, and remove all references to the Default Web Site/ECP site from c:\windows\system32\inetsrv\config\applicationhost.config

3. Re-create the CAS ECP web application: New-EcpVirtualDirectory -WebSiteName "Default Web Site" -InternalUrl "https://server1.example.com/ecp"

4. Check the Exchange Back End/ecp site is created correctly, and repair/create it if needed, as per the blog link above.