Version: [ 1, 1, 2, 0 -> 1, 1, 4, 0 ] (2016/09/26)
I'll see where are those new files, to save them
Edit: this is obsolete now as gog version became 1140.

I think you then must save 5 files (100Mo),
so you may be able to patch again the game (overwrite) even if the server is down:
. mitsurugi.exe
. mitsurugi_config.exe
. data0.kpk
. Operation Manual.pdf
. readme.txt
EDIT: So, this post should be obsolete too.

Wait, so GOG backup copy isn't updated?

The old, yes, as it was 1120. But the files I told for 1140 should be no longer needed, as gog has patched up.

Ah i see, thanks! I do wonder why updates aren't managed for this sole game though...It's the first time I experience this on GOG.

The GOG version is outdated again. Actual release is 1.1.5 while GOG has 1.1.4. GOG has been informed aboout this but apparently is not planning to update the installer even though they have all the necessary files by using the updater included in the installer.