Hi Barbara,
According to my understanding the issue is very likely to be caused by a corrupt pathdef.m file on your MATLAB search path.
To check where this corrupt file is based, please perform the steps below.
1. Type the following commands in the MATLAB Command Window
These commands will save your MATLAB path as a backup
2. Close MATLAB
3. Start MATLAB from the folder <matlabroot>\toolbox\local.
Note that the <matlabroot> part is dependent on your specific machine. To obtain the full path of <matlabroot>, type matlabroot in the MATLAB Command Window.
4. Type the following commands in the MATLAB Command Window
5. Check whether you can start MATLAB without any error message now
If the error persists, check whether a file called "pathdef.m" exists which is not the one in <matlabroot>\toolbox\local. You can check this by executing the command:
If this file exists, please remove it from the MATLAB search path.