This is now fixed. I'll leave the comment in case it happens to others.

<Edit> - Error happened again, then gone again a few hours later. Happened both at home and work, happens for a while, no apparent link between occurrences, logging out doesn't fix it.

<Edit 28Mar> - Now happening all the time. Still not fixed by logging out. I can see dclayh's page (mentioned above). This is potentially very serious if it keeps spreading.

For affected users, you can still see replies to your comments here, and possibly still http://lesswrong.com/user/<you>/submitted/, /liked/, /disliked/, /hidden/ and /drafts/.

<Edit 31 Mar> Can now access my user page after 3 days broken, but can't get to page 2 of my comments. So it looks like it's breaking trying to display some type of comment on user the page, and fixed when said comment dropped to page 2. Maybe a certain combination or nesting of editing flags confuses it?

<Edit 31 Mar> Broken again, but can access all pages that don't list my comments. The only comments I made/edited since it worked are this one, this one, this one and this one, but clearing the comments out doesn't fix it.

<Edit 1 Apr> 1st page of comments accessible again, and all 4 of the above comments are on that page. Go figure.

<Edit 2 Apr> Broke again, no comments made since last worked.... ...Working again, again no comments made.