I have Opera GX not Chrome, and it's sadly not that easy to find local storage on the devtools on there, that said I did the next best thing and cleared my cache and cookies which as far as I'm aware should delete every HTML save file in my storage, the problem did not go away I still cannot get a single save file on the new Unholy Arts update, and the issue is only with that game.You have too little space remaining in your browser localstorage for html sugarcube twine games save files. Unholy arts does have sizable saves but for you to not be able to make even a single one means you probably have too many saves from other games. You can use the "save to disk" feature to get around this problem, but you could also free up some space by deleting older saves.
Here is a little tutorial (for chrome, but should be a similar process for other browsers):
1. open/select a browser tab with a sugarcube html game on it (e.g unholy arts)
2. open dev tools on that tab (f12 on chrome)
3. find the "localstorage". For Chrome, go to the Applications tab, then Storage > Local storage (which should have a single item, probably "file://")
4. after clicking that item, it should list a number of Keys, those should be be all related to save data of other sugarcube games, start deleting any you know you won't use anymore to clear up space