Internally-hosted sites started randomly going down in the past few days, but sometimes would appear.
I restarted the server, services, and did all sorts of tests.
Finally I checked the dynamic DNS, and it was no longer resolving to our WAN IP.
I found a reddit post where others are recently having this issue as well: https://www.reddit.com/r/TpLink/comments/1kpau80/is_it_just_me_or_is_tplinkdnscom_down/

At this very moment, the tplinkdns.com dynamic DNS is working again.
Obviously there's nothing I can 'fix' on this side, but this is just an FYI in case others are having problems.  Maybe I'll need to change to another DDNS provider.
But I'm -hoping- TPLink fixes this issue.