Resolution:
on Sun Solaris machine, if the input file size is large, the file Poller may not be able to read the file completely, so sometimes the error "An error occurred while parsing:  XML document structures must start and end within the same entity.." or "An error occurred while parsing:  The element type "&ltelement-type>" must be terminated by the matching end-tag "</element-type>".." will appear.

the solution is increase File Poller interval.