Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Aha!
I tried using the cscript cmd line process described elsewhere in this thread, and got a new error:
Last 5 characters of installed product key: XXXXX (they matched my MAK key, so the correct product key is already in place)
ERROR CODE: 0xC004F025
ERROR DESCRIPTION: The Software Licensing Service reported that the action requires administrator privilege.
So apparently UAC is getting in the way.
I found this thread:
which led me to this post:
Which FIXED THE PROBLEM!
The remaining question is, Why is UAC suddenly putting itself in charge of activation *after* SP2 when it did not appear to get involved previously? I made no changes to UAC on any of the machines where SP2 was installed; I try to avoid touching UAC as much as possible.