From Red51 in another thread about issues with the map
"It seems that there is a problem if a high map resolution is used - apparently there is a resolution of 4096 set for map tiles (in the server.properties). We will fix this issue with the next update
Please try this: First, shut down the server. Then open the server.properties file and change Settings_MapTileResolution to 1024. Now go to the "Worlds" folder into the particular subfolder of the server world and delete the "Map" folder there (it contains the already generated map tiles, which are broken in this case). Restart the server - the map should work properly now "