Nearly a week later, this is the response I received from the Steam helpdesk:
Thank you for contacting Steam Support.
Technical support for this title is handled by a third party support department - please follow the instructions below to contact the support provider to troubleshoot this issue:
Title: Dungeon Defenders
Please let us know if you experience further difficulties, or if you have additional questions or concerns.
The Dungeon Defenders forums FAQ thread states:
Q: Steam achievements are not viewable and not working
A: Valve appears to have some global issues with their Stats/Achievements severs and are working to get this fixed.
So I've basically been sent in a circle, seeing as Steam is telling me to talk to Dungeon Defenders and Dungeon Defenders staff say that the issue lies with Steam.
However, according to this thread and a note that has now appeared ingame since it has been patched post-release:
Q: My achievements aren't working!?
A: Achievements are only awarded while playing in ranked games.
The solution I have found to enable viewing my personal achievement page is to acquire an achievement in Ranked mode, then your personal achievement page will be properly visible and stop redirecting you to your profile.