I've got a brand-new 2016 environment that is having some authentication issues. Currently, I cannot open the (only) site in SharePoint Designer, nor can I edit its forms in InfoPath. When I try, I'm delivered a blank error message (as described in this blog) and Fiddler records multiple 401 responses.
This seems to be a common problem, but the wealth of answers out there have not worked for me. So far I've ensured that IIS has proper authentication providers enabled, enabled anonymous access in CA, disabled the loopback check in the registry, set up AAM to accept the hostname and the FQDN, and ensured several options in the
web.config aren't interfering (as referenced by posts like this one and numerous others around the internet).
What next steps can I take? ULS logs are just showing a generic "Access denied" message that doesn't help, and I've tried multiple accounts that are owners of the site and/or site collection. I'm a little stumped at this point.