I have looked through several other posts on this topic, but all were at least a year old, if not much older, and none had replies with effective suggestions.
In the instant case, I have a Veracity TimeNet Pro GPS Clock server being used to serve NTP time to IoT devices.
The PING sensor for the device works without issue, and it is up and accurately serving time to both IoT devices as well as Windows servers.
However, I continually get an error on the SNTP sensor that reads "The server did not return a valid time (code: PE085)".
When I look at the management page for the device, there are no current leap second warnings, so I'm unsure what could be throwing PRTG off. Unfortunately, installation and use of WireShark here is prohibited for a number of reasons, including security requirements at the customer site we've configured this for, as well as some issues we've seen in the past when installing WireShark on a probe device (which is the only route of remote access we have to this facility, which, I should also note, is a nearly 3 hour drive from my office), not to mention the fact that the NTP server itself is a hardened, access limited, purpose-built device, and not something running Windows or Linux that I could run Wireshark on/from directly.
I'm hoping maybe there's some more institutional knowledge available gained in the last few years since the previous posts I've read, and that someone may be able to offer some other advice other than "Wireshark", which is impractical here.