guess this is what lead you to believe the error was on secure.domain.com but it isn't, sorry!
I have asked you Url of 404 error, but you not reply me.
but the ajax support have said this is down to easyprofile
this is not true, redirect with itemid to external URL not work in Easy Profile as Ajax login because is not possible with Joomla API. so no easy profile related. they give you a bad support.
Anyway server side redirect is impossibile to work with your sync user (between different domains) component because it sync with JavaScript. we have point you in right way, this had to be done by the support of this sync software.
you always tell me that your problems are related to Easy Profile, but it is not!!
this redirect probably become from some redirect configuration(made by you):
- Ajax login redirect after login parameter
- easy profile redirect after login (and first login) parameters
- modules redirect options
- login menu item redirect parameter
- some php code forgot(in your redirect tests)
you need to check all these things made by you on both sites www and secure.
I can't check because we can t replicate this error. this is a courtesy because is not related to some bug of Easy Profile.