200010 warning at repeatedly run stop task in a loop

Hellow everybody,

I keep getting the error 200010 after I run the attached code. The strange thing is that the code will run normally at first and this warning will only repeatedly appear after about two or three minutes.

I want to control the working timing of this signals generation task, as there are other tasks needed be done before the next group of this signals is generated. Thus here in this loop, the start() and stop() function is been run repeatedly. Sleep time is for other tasks' woring.

What am I doing wrong? Any other functions or judge needed to be done in this loop? Or if there are other methods better applicated for this task?

Thank you in advance and have a nice day, everyone!

Nicolas