How to re-add a deleted VSTO Add-In

I believe that you need to change the version of the VSTO add-in in order to be able to update it or re-install it. Once an add-in version has been disabled, this is permanent.

See Uninstall MS Word Add-in for details. Let me know if that worked.

What is the LoadBehaviour in the registry? It should be 3...

To find out, look in

• 32bit Word => Root\Software\Microsoft\Office\Word\Addins\add-in ID
• 64bit Word => Root\Software\Wow6432Node\Microsoft\Office\Word\Addins\add-in ID

where Root is HKEY_CURRENT_USER or HKEY_LOCAL_MACHINE, depending on whether the addin is installed for current user or everyone.

If the value for LoadBehaviour is not 3, then change it to 3 and restart Word.

Here is the Microsoft docs about it: Registry entries for VSTO Add-ins

If it still does not work, try to re-enable as it might be hard disabled:

• In the Manage box, change COM Add-ins to Disabled Add-ins, and click Go.
• Select the add-in and click Enable. Click Close. Now the add-in can be loaded again...
• Back in the Manage box, change Disabled Add-ins to COM Add-ins, and then click Go.
• Check the checkbox next to the disabled add-in. Click OK.

Here is the Microsoft docs about it: How to: Re-enable a VSTO Add-in that has been disabled

Also, if you haven't done this already, it is worth setting VSTO_SUPPRESSDISPLAYALERTS to 0 as described on this page: How do I enable VSTO error / display alerts?

If none of the above helps, then it is worth doing a Online repair of Office from Add or Remove Programs.