Jeff Pries explains how to troubleshoot various Reporting Services configuration errors:
After installing SQL Server Reporting Services (SSRS), are you receiving an Error 404, Error 400, “Invalid Request” error, or “Bad Connection” error on first visiting the SSRS web portal (the error message seems to vary based on version, browser, and whether accessing via http/https or /reports vs /reportserver) ?
I’ve run into this a few times so I’m listing the steps I’ve used to fix it. For me, the root cause of this error has been the SSRS Configuration Wizard automatically configuring SSRS to use HTTPS, but assigning an invalid machine SSL Certificate. The fix is to self-generate a new and valid SSL certificate for the SSRS website to use.
Jeff then provides step-by-step instructions.