Hmm, I don't have a site with 5.01 and https to test.
The proc that does the magic of returning you to http is
ad_get_login_url
It checks
if { [security::secure_conn_p] || ![security::RestrictLoginToSSLP] } {
set return_url [ad_return_url]
} else {
set return_url [ad_return_url -qualified]
}
So definitely check your settings on RestrictLoginToSSLP.
Next you can see if this proc or ad_return_url has any changes from 5.0 to 5.01
