Please check your SagePay plugin version. I had the same issue right after the upgrade to Magento 184.108.40.206. I contacted ebizmarts.com and they said this issue was fixed in the latest release of their plugin sage-pay-suite-pro-4.2.16. Please try updating, in our case, it fixed the problem and also helped resolving the issue which slow loading checkout page.
I would advise to disable all the custom extensions and third-party extensions. Then place a test order. If issue fixed, then enable the third-party extension one by one and check which extension causes this issue.
It looks like your disk is full.
You can also check tabel status:
SHOW TABLE STATUS FROM db_name LIKE 'catalog_product_index_price_temp'
After some diging on Magento issues: