I contacted EngineHosting and was told this:
If you log into the Enginehosting.com Control Panel where we post all server news (uses sFTP username and password) You’ll find the following information related to your troubles:
(Original - 02/24/2012) Our IPS (Intrusion Protection System) signatures are including something new for performance protection of site, and is now implemented for shared accounts (shared IP addresses) only. It is filtering out requests for any URI that starts with /member/ (This will not affect your site if you have not removed index.php from the URL, so anything with /index.php/member/ still works.)
ExpressionEngine sites with member registration activated need to change the default Profile Triggering Word setting from member to something else (http://ellislab.com/expressionengine/user-guide/cp/members/membership_preferences.html). This should be a common anti-spam practice for sites anyway.
This rule improves the performance for all shared hosting clients by filtering out a highly abused URI, here and across the Web and in the end benefits the vast majority of clients which is always the priority in a shared hosting environment.
This rule is not implemented on private IPs (for people with SSL certificates) in shared hosting, and does not affect IP addresses of VPS or private/dedicated cluster clients either unless they are finding abuse towards that URI type is adding traffic or load to their site then we can also include them also.
I can’t disagree with their decisions but I’m a little disappointed that they thought it was sufficient to post the update on a controlpanel that most users never have reason to visit.
Very uncharacteristic of EngineHosting support, which is usually top-notch. Hope they’re not starting to slip…