Subfolders Disappeared

I have a filter which moves all inbox messages older than 30 days to a subdirectory of a folder called "Old": Old/inbox. I have a similar filter for Sent: Old/sent. The filters are not automatic: I run them manually from time to time.
Recently I ran the filters, but noticed that the subfolders of "Old" were not appearing. I don't think they were there before I ran the filters, either, but I forgot to check before I ran them, and figured they'd show up once the filters ran. No.
Unfortunately, the filter log was not enabled: I've since enabled it. I tried creating a subfolder in Old, and it creates fine.
I also checked the subdirectories in my thunderbird profile directories (.thunderbird), and it looks as though those subfolders and messages are actually there: they just aren't showing up in Thunderbird. The "repair folder" option, run on Old, did nothing. Here are the file listings:
~/.thunderbird/qb0d6fox.slt/Mail/Local Folders$ file Old*
Old: empty
Old.msf: ASCII C++ program text
Old.sbd: directory
And within Old.sbd:
~/.thunderbird/qb0d6fox.slt/Mail/Local Folders/Old.sbd$ file *
Inbox: ASCII mail text, with very long lines
Inbox.msf: ASCII C++ program text
Sent: ASCII mail text, with very long lines
Sent.msf: ASCII C++ program text

So: what can you tell me?
Thanks for any help or suggestions.
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