Issue Description: Getting the "Web files are not matched with plugin version: All files need to be same version (undefined) - try refreshing browser cache (shift-reload)" error. I've run through the typical solutions people have suggested for this over the years - cleared browser cache, cleared web files, uninstalled/reinstalled, etc....nothing working. Dynmap previously worked on my server for years with no issues.

• Dynmap Version: 3.1-457
• Server Version: Spigot 1.16.5
• Pastebin of Configuration.txt: https://pastebin.com/VR3Y5v6P
• Server Host (if applicable): Anvilnode
• Other Relevant Data/Screenshots: http://54.39.68.61:8123/

I just want to update you all - I cleared out the installation again, did a manual install via WinSCP (instead of Multicraft interface) and changed the port. It seems to work now