Blocks rending as empty

The blocks on the User page, Members page and Account page render to blank. The blocks on all the other special pages render correctly (login, register, etc). I have tried it with different themes, and the problem is the same, so the theme is not the cause.

This used to work fine until a few weeks ago, then it suddenly stopped working (probably with some sort of update). I don’t have records to show which update caused it.