It is my understanding that you are getting errors when you are trying to open Add-Ons.
To resolve this please follow the below steps:
Before modifying the search path, it is important to take a backup of your 'pathdef.m' file, which can be located by using the command:
Before proceeding, ensure that you make a safe copy of the file by copying it to a folder outside of your MATLAB path.
After taking the backup, please execute the following commands in the MATLAB Command Window to restore the default MATLAB search path and rehash the toolbox cache:
For Simulink users, including an additional command may be helpful:
After this step, please use MATLAB/Simulink to see if the issue occurs again. If the issue is resolved, then you may want to save the new MATLAB search path by executing the following command:
NOTE: This will remove any custom paths you may have created. The following MATLAB Answers post is an additional resource on how to back them up:
I hope this helps you.