PS Took the new key from the new DLSite version, that was it. Thanks for pointing out where to put it,
KapitanDupa!
It seems that when a game is packaged for publishing in RM MV editor a new encryption key is generated randomly, so they differ between versions.
Well, at least it has a positive side: when the game is updated the translation should be updated too, and if you apply translation for previous version it won't work right away due to different encryption key.
The downside is, of course, that I must post new translation archive every time the game is updated, and do it ASAP.
Huge kudos to you and
KapitanDupa for uncovering and solving this problem.
translation patch isn't working for me, keeps giving me img errors, and the system.json fix from earlier in the thread doesn't seem to solve the issue either
I think that is because of the problem discussed above.
You can either copy over the encryption key from original game archive, downloaded from DLSite (open file 'www/data/System.json in any sane text editor like Notepad++ and search for "encryptionKey"; copy hex string associated with this key to your clipboard, then close the file; open same file after extracting translation archive over your game, find "encryptionKey" and paste original key instead of what is in translation patch), or download new translation archive that I'll attach to this post a bit later.
UPD: I've attached the new archive. This should work fine with version currently available from DLSite.
If it stops working, that means that the game was updated by Fuyou. Please notify me and I'll post new archive.
I'll try to publish my program for text extraction and replacement in the nearest future, but currently it lacks GUI and is somewhat cumbersome to work with, so I doubt that many people will be able to make and apply translation patches on their own.