Is this the PhotoStructure for desktops browser, or a browser on a different machine?
If your home LAN network is involved, know that I had a similar experience due to my orbi router: it had a “ddos protection” setting that dropped requests/thumbnails that I had to disable.
Even after disabling it, most thumbnails would show up, but I still had some broken images. I found that the router just wasn’t up to the task of serving hundreds of high velocity requests. I had to switch to my old Asus router running open source firmware and the thumbnail loading issue went away.
I’d first run the web service with debug logging, so you can see requests as they happen. I suspect your webserver isn’t seeing most requests.
I’d also open the developer tools in your browser, and select the network tab.
I’d also try using an Ethernet connection through a known-reliable switch rather than Wi-Fi.
I had to replace an aging cat5 cable and older switch before my nuc could reliably access my NAS. (It’s actually why PhotoStructure has import retries built-in).
If this isn’t network related, given that you’re on Windows, know that most other antivirus software can wreak havoc with PhotoStructure: only Windows Defender is supported: Antivirus Bitdefender identifies PhotoStructure as a Malware - #2 by mrm
If you’re running only Windows Defender, on localhost, and seeing this issue, please email me debug logfiles: PhotoStructure | What are in my logs and error reports?