parfeval - Struct contents reference from a non-struct array object

I changed one of my programs from serial computation to parallel computation. In serial everything works fine; in parallel also most times. When I execute my program I get the following error:

Error using parallel.FevalFuture/fetchOutputs (line 139)
One or more futures resulted in an error.

Error in test_method_time_determination (line 132)
                result_show{idx} = fetchOutputs(result(idx));

Caused by:
    Error using pi_processing_pi (line 10)
    Struct contents reference from a non-struct array object.

The strange thing is, that I get the error message just sometimes. I can run exactly the same program like ten times and once it fails and not necessarily at the same point.

I think, the problem is that the function which is processed in parallel sometimes delivers an empty result cell. My function call looks like that:

while go
    result(cnt) = parfeval(@pi_processing_pi,1,ref, target, roi, analysis.intMet, analysis.subMet, sizeX, sizeY, ns_x, ns_y, analysis.ns);
    //some changes in the input arguments
    if something break;
end

I get the results via:

result_show = cell(1,imax);
for idx=1:imax                
    result_show{idx} = fetchOutputs(result(idx));
    %some processing
end

The pi_processing_pi function evaluates the differences between two images and returns a Matrix. The functions are quite complex with several sub-functions. I don't think that it makes much sense to add all the functions. Moreover, the functions usually work and just from time to time a function evaluation fails (for a case which worked several times before). Does anybody have an idea why this problem occurs. I am using Matlab 2016b btw. But I have the same problem with 2017a.

[Andreas Thoma]

ok.. I narrowed the problem a little bit down. Apparently one of my structures "ref" which I pass to my functions fails to be created properly from time to time. When I use a breakpoint to check on ref, it is always a structure and the function runs through. But then my program never fails. But how is it possible that ref is created as a structure sometimes and sometimes not, using exactly the same code?