pathdef problem during startup

Hi,

after installing some plug-ins I have problems with my matlab 2012a: When starting I get the following messages in the command windows

Undefined function or variable 'filesep'.
Warning: MATLAB did not appear to successfully set the search path. To recover for this session of MATLAB, type "restoredefaultpath;matlabrc". To find out how to avoid this warning the next time you start MATLAB, type "docsearch problem path" after recovering for this session.
Warning: Duplicate directory name: C:\Program Files\MATLAB\R2012a\toolbox\local.

When I follow the instructions to use "restoredefaultpath;matlabrc" I'm getting warnings like

In matlabrc at 60 
Warning: Duplicate directory name: C:\Program Files\MATLAB\R2012a\toolbox\matlab\audiovideo.
In matlabrc at 68 
Warning: Duplicate directory name: C:\Program Files\MATLAB\R2012a\toolbox\matlab\imagesci. 
....

adressing many more directories like above. Besides the warnings the actual problem doesn't disappear: Due to that pathdef-problem I can't open any m-file - Instead I recieve an error diaglog saying

Undefined function 'fileparts' for input arguments of type 'char'.

Therfore, I can't use Matlab at the moment because I can't run scripts or load any file. Every help to resolve that problem is highly appreciated.