ERROR IN CUBASE LE5 (Assertion failed...)

Hi, i have problems with the Cubase LE5, when i edit a song using this program or i export a song appear a window with the next text:

Assertion failed: next_event->time_stamp>=current_sample_clock, file…\se_sdk\SEModule_base.ccp, line 203

Please someone tell me what is happening. i done almost everything to fix it, but i don’t have the right solution for this yet. Thanks for your help.

Note: i am using windows 7 and sometimes it seems this error happen when i am working on cubase with VST instruments.