That could very well be a flawed sample. It could also be a RAM issue, or you are attempting to use oversampling with a vst, that does not like it.
First:
* use save-as and resave the project with an alternative new name
* shut lmms and then the computer down
* wait 2 mins, then restart the pc, then peak in precesses, it you have c*** running (ICQ MSN etc ) -Kill it!
Open LMMS and load the alternative project
Go into
Do you use oversample? Dont!
* Export
If it fails, i need to see the project