When I do /dynmap fullrender its starts rendering and stop after 10 seconds claiming thats the render is done. and its a brand new map that I downloaded and uploaded on the server and loaded it first on my singleplayer so its not a map generated by the server.

Sounds like a WorldPainter render issue, substitute EarthMC for Worldpainter:

The EarthMC map has many known issues with Dynmap such as, incomplete render, only rendering where players have visited, or not rendering leaves and other foliage.
This is a result of how the map was made and is not an issue with Dynmap, luckily there is a fix.

1. Download the WorldBorder plugin: WorldBorder
2. Pause Dynmap: /dynmap pause all
3. Run worldborder with the force flag: /wb [worldname] fill [freq] [pad] [force]

• [worldname] is the name of your world folder
• [freq] is how many chunks a second are generated, 20 is the default. WARNING: setting this too high will cause server lag
• [pad] how many chunks beyond the border to generate, this makes sure that if a player visits the border, all chunks are already generated. The default is 208, I recommend setting this to 0 for EarthMC
• [force] This must be set to true

4. Wait for worldborder to finish
5. Unpause Dynmap /dynmap pause none, you may have to restart the fullrender using /dynmap fullrender [worldname]

+1, having the same issue as well. Using Paper Build #284 and Dynmap 3.0 RC3

[07:28:58] [Server thread/INFO]: This server is running CraftBukkit version git-Spigot-2f5d615-aae46f8 (MC: 1.15.2) (Implementing API version 1.15.2-R0.1-SNAPSHOT)

dynmap version
[07:25:20] [Server thread/INFO]: Dynmap version: core=3.0-SNAPSHOT-338, plugin=3.0-SNAPSHOT-338
dynmap fullrender Bulgraria:Surface
[07:20:23] [Server thread/INFO]: Full render starting on world 'Bulgraria'...
[07:20:23] [Dynmap Render Thread/INFO]: Full render of 'Bulgraria' finished.

...and yet the surface map has zoom-out anomalies (black where other zoom levels of the same region are not black, and map edges that are missing where manual radiusrender was used to to fix render anomalies). Just prior to this, flat and cave maps fullrenders ran and completed as expected.

Even weirder:

dynmap purgemap Bulgraria surface
[07:37:43] [Server thread/INFO]: Map tile purge starting on map 'surface' for world 'Bulgraria'...
[07:47:39] [Dynmap Render Thread/INFO]: Purge of tiles for map 'surface' for world 'Bulgraria' completed
dynmap fullrender Bulgraria:Surface
[08:14:25] [Server thread/INFO]: Full render starting on world 'Bulgraria'...
[08:14:25] [Dynmap Render Thread/INFO]: Full render of 'Bulgraria' finished.

What! Now I can't regenerate the surface!

dynmap pause all
[08:32:54] [Server thread/INFO]: [dynmap] Full/radius render pause set to true
[08:32:54] [Server thread/INFO]: [dynmap] Update render pause set to true
[08:32:54] [Server thread/INFO]: Full/Radius renders are PAUSED
[08:32:54] [Server thread/INFO]: Update renders are PAUSED
[08:32:54] [Server thread/INFO]: Zoom out processing is ACTIVE
dmap mapdelete Bulgraria:surface
[08:33:09] [Server thread/INFO]: Refreshing configuration for world Bulgraria
[08:33:09] [Server thread/INFO]: [dynmap] Loaded 2 maps of world 'Bulgraria'.
[08:33:09] [Server thread/INFO]: If you are done editing map data, run '/dynmap pause none' to resume rendering
dmap mapadd Bulgraria:surface title:"Bulgraria ISO" perspective:iso_SE_30_hires shader:stdtexture lighting:shadows
[08:58:05] [Server thread/INFO]: Refreshing configuration for world Bulgraria
[08:58:05] [Server thread/INFO]: [dynmap] Loaded 3 maps of world 'Bulgraria'.
[08:58:05] [Server thread/INFO]: If you are done editing map data, run '/dynmap pause none' to resume rendering
dmap mapset Bulgraria:surface order:2
[08:59:29] [Server thread/INFO]: Cannot edit map data while rendering active - run '/dynmap pause all' to pause rendering
dynmap pause none
[08:33:30] [Server thread/INFO]: [dynmap] Full/radius render pause set to false
[08:33:30] [Server thread/INFO]: [dynmap] Update render pause set to false
[08:33:30] [Server thread/INFO]: Full/Radius renders are ACTIVE
[08:33:30] [Server thread/INFO]: Update renders are ACTIVE
[08:33:30] [Server thread/INFO]: Zoom out processing is ACTIVE
dynmap fullrender Bulgraria:surface
[09:00:24] [Server thread/INFO]: Full render starting on world 'Bulgraria'...
[09:00:24] [Dynmap Render Thread/INFO]: [dynmap] Loading default resource pack
[09:00:59] [Dynmap Render Thread/INFO]: Full render of map 'surface' of 'Bulgraria' in progress - 100 tiles rendered (349.13 msec/tile, 311.57 msec per render)

Wow, surely that was harder than it should have been...

might be related to #2915

maybe related to #3003