I find the map takes a while to get everything loaded up - even on a fat pipe at work it takes a good few minutes to display it all. Maybe you're not being patient enough?
That square's been ok for me too.
The reason for DynMap issues could be that the build I'm using hasn't been updated for the current Bukkit build.
I've read that they're making a new "HD" version or something.
Tried the XP equiv "Junction Link", tried mapping the folder as a drive, tried a direct alias to the full UNC address.
Web Server and Minecraft server are on different systems.
Linux: ~/.minecraft/screenshots Windows: %appdata%\.minecraft\screenshots Windows Vista \ 7: ~:\Users\USERNAME\AppData\Roaming\.minecraft\screenshots Mac OS X: ~/Library/Application Support/minecraft/screenshots