So this schematic got me looking... This doesn't seem right: [img:center]https://photos.app.goo.gl/tf78cj6XewCQWWU17[/img] and specifically, [img:center]https://photos.app.goo.gl/D3yvHt1ULpf9Wao88[/img]

It looks like solder paste or some kind of cap leakage seeped under, but there are no obvious sources anywhere near it.

I've flushed with 97% Isopropyl and then some DeoxIT D5 using a 0.3mm syringe and a wee bit of scraping. A flush and then blew it out with air. Looks better, but waiting for it to dry before try again.

<some hours later...>
Same result. Gets to 24% and errors. I don't know what that is under that first row of pins--it is not on any other device on the boards and nothing was ever spilled in there. I've been very careful to treat these with great care even when not in use. Maybe it is not related.