Why does iTunes briefly show an empty popup window when accessing online features, only on one of my computers?

[msephton]

When I choose any online part of iTunes a window pops up for a second or two, is blank apart from an indeterminate-style spinner, and goes away after a second or two.

- This happens on my Mac mini (2018), but not my MBP (early-2013)
- Mac mini is 3X or 4X faster than the MBP
- Mac mini was a recent Migration Assistant transfer of the MBP
- I have not installed any unique/new software on the MBP since
- Both are on 10.14.6 Mojave

Things I have tried:
1. logging out and back in on iTunes
2. de-authorizing all computers connected to my account

Neither worked.

Any ideas about the following:
1. what is this window?
2. how to stop it appearing?