I followed How to: Configure SharePoint Integration on Multiple Servers step-by-step to configure SharePoint Integration on two servers. But on the step "Set server defaults" of "Configure the Report Server Integration Feature in SharePoint Central Administration", I only saw this error:
An unexpected error occurred while connecting to the report server. Verify that the report server is available and configured for SharePoint integrated mode.
I was sure I followed the steps carefully. Then I tried to look for error details in Web and Database log files, but there was no any error message. After many hours of investigation, I found somebody mentioned Kerberos/NTLM on MSDN Forum. As my environment did not allow me to use Kerberos, I tried to set up NTLM on both SharePoint server and Report server:
cscript adsutil.vbs set w3svc/NTAuthenticationProviders "NTLM"
Then I was so happy to see the error disappeared! :)