For some reason it would appear that your ONLINE table isn't being purged often enough. It looks like it's possibly around 16 megs in size, which even on an extremely busy board it shouldn't grow that big.

I'd run 2 queries. First see how many rows are actually in the table with this:

SQL Query
select count(*) from ubbt_ONLINE

Then purge everything in the table

SQL Query
delete from ubbt_ONLINE