Just spent 30 minutes on the phone with web host tech support. He went in and said there is no limiting of XMLRPC, nor is Sucuri firewall in any part of this – as it is not installed in my site/hosting server. He loaded our site on multiple devices on his end and did not get the error, and it seems that I can’t cause the original error on any browser – so some kind of transient/intermittent issue?
I also forwarded the last message regarding limits and to check with web host. That was a dead end as, according to them, none is applicable to our hosting.