It seems you have added an index yourself on 'lastactivity', as this is not a default vB index. This index might be corrupted, i suggest dropping the index. If all s working as it should, you could try adding it again.
Also this is not a default vB query, as vB nowhere in the source uses FORCE INDEX, i suggest removing that code.
What was the other half? I ran the query and posted the out put.
I have all original vb files intact so I can;t remove it.