OPCEnum is part of the "OPC Foundation Core Components" which OPC Toolbox relies on. You can use the "opcregister" function in MATLAB to try to repair or (re)install these components. See the following page in the documentation for more information about the "opcregister" function:
You may access the same page locally by typing the following at the MATLAB prompt:
Note that you may need to start MATLAB with elevated privileges to be able to use "opcregister" correctly, to do so start MATLAB by right-clicking on the MATLAB shortcut and selecting "Run as administrator".