I may have fixed the issue on my laptop in the stupidest way imaginable.

It appears that deactivating every single plugin on my site is allowing me to post on the new laptop.

It is allowed as a mental exercise for the reader why changing something at the site end that does not live on this laptop allows me to post on this laptop, but everything works fine on my desktop, which has the same system software, the same network, and the same applications.

I’m going to go to bed and read.