Hey, I've just tried again and didn't experience any issues. I downloaded steamcmd on a new Windows machine, did a clean install of the latest public Rust server branch through it and used the map file uploaded on codefling. I didn't install any plugins, extensions or oxide/carbon. The only thing I have in my .cmd/.bat file to start the server with is this:
C:\steamcmd\steamcmd.exe +force_install_dir c:\rustserver\ +login anonymous +app_update 258550 -beta public +quit
RustDedicated.exe -batchmode +server.port 28015 +server.level "Procedural Map" +server.levelurl "<put your url here>" +server.maxplayers 10 +server.hostname "Urban's Custom Maptests" +server.identity "customserver1" +rcon.port 28016 +server.seed 1234
The only thing I can still think of is that your url from dropbox doesn't work like it's supposed to. This is the documentation I found on how to use dropbox to host a rust map file:
"You can use a service like Dropbox to upload your map file and generate a direct link. Simply upload the file to Dropbox, then create a share link by clicking the "Share" button followed by "Create Link". Paste that generated link into your server.cfg in the server.levelurl field, replacing the ?dl=0 with ?dl=1."
Also make sure you unzip the file.
I hope this fixes it for you, please let me know.