GUI running on R2009b, but not on R2011b.

I have a Simulink model (which it was not developed by me) and which includes two GUI's. This model was developed in the R2009 version of MATLAB.

I have the following problem: When I run the Simulink model using the version R2009b the GUI's work fine. When I try to open it with the R2011b version the GUI's window don't even open and I get the following error:

Error due to multiple causes. -->Error evaluationg 'OpenFcn' callback of < name of GUI > block (mask) < name of model/name of GUI >. --> block_diagram does not have a parameter named 'Simulation'.

Any ideas? (I must admit I know nothing about GUI's)