first try if this is a generic problem:
open a new project
add say 10 notes to the default 3oc in the songeditor and export that 'project'
If this fails, it is proven that generic exports are faulty.
If it is a succes, then it is most likely that you have a vst in the problem-project that causes the crash -Or, you do not have enough ram for the process
You should then try to shut everything down except lmms
-next step is that you tell us the compleete tech spec of your system, but try the abowe first.
Br.