Magento 2 developer support

I have been building a new Magento 2 website for my business, with data migrated from my old Magento 1.7 website by a third party company. I have run into a problem with Checkout where, after adding a product to My Cart and clicking on Proceed to Checkout, I get just a blank page with the company logo, nothing else. I can see errors in the Chrome console like below Uncaught TypeError: Cannot read property 'after' of null at [login to view URL] ([login to view URL]) at [login to view URL] at [login to view URL] (<anonymous>) at [login to view URL] ([login to view URL]) at [login to view URL] at Registry._resolveRequest ([login to view URL]) at [login to view URL] (<anonymous>) at Registry._updateRequests ([login to view URL]) at later ([login to view URL]) You can see for yourself at the website [login to view URL] The same problem happens both with Magento Checkout and IWD Checkout Suite. I have tried disabling unused shipping modules (Temando, USPS, Fedex etc) but this has not worked. I would like a fixed price to fix this problem from a good and reliable developer that I can work with also for ongoing support. Full access to Magento Admin Panel and SFTP/SSH will be provided.