Update-woes continue. A recent OTA (OPR6.170623.017) for the Nexus 6P appears to have broken SafetyNet and, subsequently, things like Android Pay. Thankfully, there's a fix. All you need to do is update to a slightly newer build (OPR6.170623.019), and we've even got a link if you'd rather not wait.
The problem was reported on Google's Product Forums, and just yesterday Google promised to resolve the issue with a corrected OTA. Anyone that is currently running the September security update via build OPR6.170623.017 is likely affected. If you manually flashed that build and noticed the problem, you'll want to migrate to OPR6.170623.019.
Reports differ, but it seems that .017 was never distributed outside Google's OTA downloads and factory images site, so most people should not have been affected. If you manually flashed the September images from one of those sources, though, you'll want to check your build number.
The official pages for both OTAs and factory images don't yet list the .019 update, but we've got a link for a direct download from Google's servers. If you don't want to wait for the OTA to hit your phone, you can sideload it yourself via the instructions at the top of Google's OTA page.
Google has replaced the .017 images with .019 at both the OTA downloads site and the factory images site, so you shouldn't end up with the broken images by accident, should you flash one of them. If you needed the full images for some reason, or you just don't like sideloading OTAs, you've also got the option of full factory images now.