Mailbox titles blank when they have unread items in them

[bjenkins]

Since the upgrade to 6.05, our mailbox titles are blank when they have something unread in them, ie when they are bold. As soon as you read everything in them, the title returns to normal, and is readable. This only happens in Ajax mode, and in any skin. I thought maybe it had something to do with this bug:

Bug 44673 – Viewing calendar w/ ajax is blank in 6.0.5

...which fixed our calendar, but leaves the mailbox titles blank. What could be amiss?