You may have a problem in your Matlab path, FieldTrip is not using the correct isfolder function.
What is your version of Matlab?
What is the output when you type which isfolder in the Matlab command window?
Try the following: edit your Matlab path, remove all the folders that are not related with the MATLAB installation, including the Brainstorm and Fieldtrip folders and subfolders (they are not supposed to be added permanently in your path).
Restart Matlab, start brainstorm.m, try again.