Hm, getting something similar targeting a specific forum with an "entire posts" search:
Warning: assert() [function.assert]: Assertion failed in /sphinxapi.php on line 284
Query failed: searchd error: index 'vbpost': sort-by attribute 'lastpost' not found.
Again, could be my currently messed up setup, it is hard to tell until I get the upgrade done tonight
Edit: haha didn't see that something else was broken. I'll leave the testing to you hehe