Simulink: Undefined function "fieldnames" for type double

I am unable to open many Simulink blocks (even simple things like Scope, etc) when this error pops up.

I'm not sure whether this is relevant but

which fieldnames

returns

(C:\Program Files\MATLAB\R2013a\toolbox\matlab\datatypes\@struct\fieldnames) %struct method

and I suspect that for some reason Simulink may be trying to use this fieldnames method instead of the one for Simulink? Is this is the real problem, how can fix it?

Thanks in advance.

EDIT: For some reason this only happens when I navigate to a certain folder in MATLAB. As far as I know there isn't anything special about this location...