Why checkbox/popup mask parameters modified in callback don't work

I was tasked to modify S-Function to take a few more parameters, in ML2013b. Then for that block I defined new mask parameters, processed them in StartFcn callback and passed them to S-Function. The problem arises when simulating that block as I was getting error: 'Error evaluating 'StartFcn' callback of block(mask)...', caused by cannot modify parameter 7 of S-function ...

As the process of the development happened to be iterative then I experienced quite annoying situation as those parameters could work for a while then after a modification of mask again they stopped working then either good luck or magic with subsequent tries of refreshing the block and model(Ctrl+K, Ctrl+D) made them work again. So, it is something I had missed or some restrictions I am not aware of while using the mask parameters. I want to say I already have 8 parameters in my mask some of them are Edit, Checkbox or Popup type, parameters in mask are Tunable, Enable, Visible.

Has someone experienced such a problem or know how to explain that bahaviour?