Error while running test in Loadrunner Controller

Hello,

I am using windows server 2008 r2 and loadrunner version 12.53.

After I start a test on loadrunner controller, I saw an "Assertion Failed!" error(Program: Wlrun.exe, File: srv\Vuser.cpp). I start the test in controller, it run without any error for 10 vuser and then this error appears.

I have also attached the screenshot of error.

can anyone tell me, how to fix this error?

Thanks