I too had/have a quite similar problem as Jon. As a workaround, I was able to individually install 5 of 6 the required downloads via the “Purchased” pane.

The sixth item (iTunes) was initially missing from the Updates pane, so I thought that would need be separately downloaded & installed. But after I installed the first 5 items via the Purchased pane, iTunes magically appeared in the Updates pane and downloaded as usual.

I haven’t a clue what is going on, but I suspect server issues at Apple. confused

Harv
27" i7 iMac (10.13.6), iPhone Xs Max (12.1)

Those who can make you believe absurdities can make you commit atrocities. ~Voltaire