I am using WP Download Manager plugin to serve PDF document previews on my site straight from Dropbox. The plugin protects against download link sharing by changing the document links every 4 hours. However, the links worked well for about 4 hours before returning an error message all the time, as follows:
[login to view URL] v1.9.426 (build: 2558a58d)
Unexpected server response (500) while retrieving PDF
I need help with fixing this, please.