When other add-ins are installed (such as the Article Authoring Add-in or Math Add-in for Microsoft Office Word, for example), the ontology add-in may not load. This problem can also happen with other add-ins.
The current belief is that the problem is related to this:
From the description “this issue only reproduces when you have another Add-in installed that implements IDTExtensibility2 interface … this is related with one of the CLR bugs and the only way to avoid this is shiming the COM Add-in For e.g.(The Microsoft Math
Add-in for Microsoft Office Word 2007) using the COM shim wizard and register the shimmed dll on the problem machine)."
Changes will likely have to be made to this and other add-ins to avoid this problem.