I've the exact problem. Do you find any solution to this?
This is probably due to the Default-Web-Path entry in the ar.cfg file. This is a hard coded URL of the Midtier uses for emails/alerts by AR System processes.
Single Sign On for BMC AR System
But it is working for one midtier where it is pointing the correct URL (with MidTier Server). But this is not the case with another midtier. Ofcourse, in my AR Server's ar.config I hardcoded Default-Web-Path to my primary midtier server.
I found the answer but had to confirm with positive results before posting. Well I just now had the problem again on a test server and fixed as below.
I updated Midtier/WEB-INF/viewer.properties. There is a parameter in there called "base_url" which is commented out by default. I changed it to:
Hope this helps!