@TheDog
Thank you for noting location of LOG file.
Observer is set to false.
26343 13:08:20.484 [pool-4-thread-1] ERROR o.t.game.client.HeadedGameRunner - java.lang.NullPointerException: Cannot invoke "java.awt.image.BufferedImage.getWidth(java.awt.image.ImageObserver)" because "fromFile" is null
at games.strategy.triplea.image.TileImageFactory.loadUnblendedImage(TileImageFactory.java:220)
at games.strategy.triplea.image.TileImageFactory.getImage(TileImageFactory.java:132)
at games.strategy.triplea.image.TileImageFactory.getBaseTile(TileImageFactory.java:116)
at games.strategy.triplea.ui.screen.drawable.BaseMapDrawable.getImage(BaseMapDrawable.java:17)
at games.strategy.triplea.ui.screen.drawable.MapTileDrawable.draw(MapTileDrawable.java:37)
at games.strategy.triplea.ui.screen.Tile.draw(Tile.java:72)
at games.strategy.triplea.ui.screen.Tile.drawImage(Tile.java:57)
at games.strategy.triplea.ui.panels.map.MapPanel.lambda$paint$9(MapPanel.java:779)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
at java.base/java.lang.Thread.run(Thread.java:1583)
The error seems to stem from a baseTile being null. All are good. Including reliefTIles. The map does have a width that aligns exactly to the 256x256 tile size, thus 20x20 tiles, name 0_0.png through 19_19.png. (5120x5120 map width/height in properties) The small map in upper right corner does not display right and bottom edge correctly.

The border slicing may have created zero area islands, island chains, and two-pixel spurs. Only noticed two candidates for that, but they display correctly. Get observer error, even with single polygon boxes around territory centers, hasRelief set to false (reliefTiles folder must be removed to not display reliefTiles). I'm going to recreate the XML script from scratch. Maybe missed an initializer, version, or other element that is not optional.
Note: map/smallMap.jpeg is 300x300. Is it supposed to be a particular scaled size of main map (which is 5120x5120)?