max_execution_time would be why the rebuilders fail. A slow server is having problems processing the rebuild "chunk" within the max_execution_time period. Either update the max_execution_time to 60 or 90 if you cannot move to a standard server.
I'm going to go out on a limb to make a guess that;
A) the forum is on shared hosting, where the processor is being throttled.
B) Or is given very little ram to handle the data "chunks."
C) Or just really old hardware.
