Always bare in mind that when stating "we've only changed", this is by very definition custom. Its always worth checking those items first, as you have seen there. The simplest way to check first of all is by going to support in the admin CP, clicking customisation overview, and clicking disable. You can then open your site in another tab and check if the issue is present. You can then click to enable again once done. Its just a quicker way for you to check