@Tague-Carlyon

the event loop of QCoreApplications needs to be running, otherwise this won't work at all.

@imhs
I figured it out!
The problem was in the read function function where I call it every time toggling the run switch.
Inside that function there is a timer which aims to read the registers and update the UI. Every time I call that function a new timer was generated. Therefore after sometime many timers was running in parallel which caused the problem.
Take care about timers!