Missing subforums
 

Last night my server crashed, then when it woke up vBulletin spat out "Table forum is crashed and should be repaired" error.

So I do. I repair the tables like it says.

First time round:

vb_forum rows changed from 171 to 169

Second:

vb_posthash repaired, row count changed from 16 to 15

Restart mysql, restart nginx, forum is back online. A quick glance and everything seems fine so I go to bed.

I wake up the next morning to find missing forums.

One forum category along with all of its children and one subforum at a depth of three.

If I visit a link to where the forums used to be, I get a "Invalid Forum specified. If you followed a valid link, please notify the administrator" message.

1. Is this related to the table 'forum' shrinking from 169 to 171?
2. What content do the tables 'forum' and 'posthash' contain?
3. Is there any way to fix this?

Things I've already tried:
1. Repair.php
2. Rebuilding forum indexes.