The A/B test JS file is missing
After the initial installation you might notice that, if you checked the A/B testing feature when including the tracking script, the domain.ab.js file is missing, returning "404 Not Found".
This happens because the AB testing file is only generated once at least one AB test has been created and published.
Go to Domains->Wrench icon (for the domain you want to change the tracking snippet) -> Include the tracking pixel.
Disable the A/B tests feature and copy the tracking code. Update the tracking snippet on your domain.
Removing the A/B test code from the tracking snippet
B. Create and publish an empty test
If you do plan to use A/B tests but don't want to create one yet, you can still generate the A/B test file by going to A/B tests -> Add new test (bottom right) -> Publish Tests
Creating an empty A/B test to generate the missing ab.js file