SQL Reporting services publishing through ISA

Hello colleagues.

I have SQL 2012 Reporting Services. Into internal network when I in internet explorer go to https://reports2.domain.ru/reports - all fine. I publishing reports2.domain.ru to External via ISA 2006. Sharepoint don't used.

From Internet I go to https://reports2.domain.ru/reports, in ISA authentication window I input my login and password andsee an error: "Error code: 500 Internal Server Error. The target principal name is incorrect."

I see similar questions on technet, but I don't find solve of my problem.

Please somebody help me!!