I launch iTunes, choose Account > View My Account…, and authenticate without problem.
Then, next to the date of my most recent purchase, I click See All, and I get…a spinning pinwheel ad infinitum.
I can immediately view the same information in the App Store on my iPhone, so something must be broken somewhere. I wonder if it will be fixed, or if Apple is nudging holdouts to “upgrade.”
Same issue here - spinning wheel with iTunes & Mojave
Also Manage Payments (eg credit card expiry) and other edits to the account result in the spinning wheel. 
Fortunately I have another Mac running Sequoia for managing these things.
I have submitted a bug report to Apple but, in the unlikely event of a response, they will probably tell me to update the macOS!!
Glad to know I’m not alone. At least I can stop trying to identify variables. 