Issue

When I start Outlook or when I try to activate the Rainbow Outlook add-in I'll have following error message and the Rainbow Outlook add-in is set (or stays) disabled.

"Installing Office Customization : There was an error during installation"

Root Cause

This issue might be linked to the way Microsoft ClickOnce trust prompt has been configured on the PC.

Read more about this here

How to fix the issue

You'll have to change the TrustedSites PromptingLevel in the Windows Registry and set it do Enabled

With the registry editor you'll have to search the key TrustedSites located at:

Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\Security\TrustManager\PromptingLevel

And change the value from Disabled to Enabled