Minecraft 1.18.1
Carpet Mod Version "fabric-carpet-1.18.1-1.4.57+v220119"
Steps to produce:
Try to type in the /script download argument, and there will be no tab completion for the command.
Enter a full command, example "/script download survival/nopickup" and the server will freeze until a Watchdog crash happens.
This happened for me with multiple different scripts and considering there is also no tab completion it is probably not being caused by a specific script.
Crash report-
crash-2022-03-27_01.33.39-server.txt

Not having tab completion seems to indicate there'd be no connection to github... though it seems like it's blocking on the repo structure creation, where maybe the lock wasn't removed successfully after a failed connection?

Do you have the full logs? If so, can you please post them here?
Notice that you are running on a dedicated server. Does this issue occur if you perform the steps client-side, as follows?

1. Install Carpet mod on your client.
2. Start the client.
3. From the main menu, click "Singleplayer", then create a new world or join a world with cheats enabled.
4. Start typing in chat: /script download (note the trailing space)
5. Take note whether there is tab completion or not.
6. Finish typing the command, such as /script download survival/nopickup
7. Run the command.
8. Take note whether the server hangs or not.
9. After a while, hold F3+C for ten seconds to crash the game.
10. Post the resulting logs and crash report here.

Finally, does this occur in 1.18.2 and/or 22w12a?

I know I had fixed something in the app store manager back in Jan but am unsure if it is related or if my changes made it into a release. You would see almost this behavior before my changes if you had tried to load scripts, then switched levels, then try loading again. But this could also be connectivity (not sure what the standard URL timeout is), as mentioned.

Well, in my case I didn't see a crash IIRC. Specifically, the issue I saw (and fixed) was:

1. Start Minecraft and load any world
2. Use script load <whatever> (autocomplete works)
3. Save world and return to title screen
4. Load another world
5. Now script load never autocompletes

So in that case, the workaround was to quit Minecraft completely and then go into the second world and it would work again. But this sounds a bit different than what you are seeing, perhaps.

Just pushed a commit that should prevent any race condition in that area (which I'm quite sure causes this), will be in next release.

If it still happens after that though please open a new issue.