Cannot create Azure subscription (NoAzurePlanFound) - locked 2024 invoices can't be paid in portal, Contact support is empty

I can't create any Azure subscription on my Microsoft Customer Agreement (Individual) billing profile. Every attempt, from both the portal and the Azure CLI (az account alias create), fails with: NoAzurePlanFound: Not all Azure plans are enabled / active for this billing profile. Please contact Azure Support.

What I found:

• The Billing API shows three invoices from April-June 2024 in Locked status, totalling about 0.72 EUR (one of them is G044264883, 0.31 EUR). All earlier invoices are Paid.
• When I try to pay G044264883 in the portal, it says: "You can't pay this invoice in the portal. Please contact Azure Support for assistance."
• Yet the billing profile Overview shows "Amount due 0.00 EUR - No payment needed".
• az billing profile show reports the profile Active, with Microsoft Azure Plan (skuId 0001) listed as enabled.

What I've ruled out:

• I am Owner at the billing account, billing profile and invoice section scopes, signed in with the correct work account and tenant.
• I accepted the updated terms. Billing alerts are 0.
• I replaced the payment method with a valid Visa and retried: same error (six attempts in total).

Why I'm posting here:

• In every portal support entry point (Help + support, the billing account's New support request, the Cost Management support blade), the Contact support tab is empty or the button does nothing.
• az support no-subscription tickets create returns "InvalidSupportPlan: Your support plan type is Free".
• The phone line for Spain (800 000 024) did not work for me either.

What I need: someone from billing support to unlock and take payment for the locked invoices and re-enable the Azure plan on my billing profile, or to open a billing support case for me. I can share my billing profile ID, invoice numbers and tenant ID by private message.