I'm happy you enjoy the game.
From your comment I read, that the progression (delay between difficulties) was ok in terms of time, but it felt unfair in terms of how much points you were getting for solving bigger levels.
I will add this balancing tip to my to do list.

When it comes to hints for the final difficulty it was a tough choice, because these levels are a bit different than the others. By being different I mean there is still only one possible solution, however there's no algorithm that is able to solve them. It would be possible to give some hints, but there would be spots where the hint would say "sorry, I don't know what to do next". So I decided to treat the last difficulty as "uncharted territory" and let the player prove he/she is better than machine :)

But, I get now that sometimes players prefer to have incomplete hints than none at all.

What browser do you use?
Was it in incognito mode?

Edit: Thanks for providing the details. Thing is, I tested it on Chrome, Firefox and Edge without being able to reproduce it, but other players also reported this problem. Maybe some day I will find the root cause. For now I can advise a mobile version instead, from Google Play Store (if you use Android).

Glad you like the game.
Regarding the error, I get it on my Firefox version as well and unfortunately it's something that Firefox has to fix: https://bugzilla.mozilla.org/show_bug.cgi?id=1678243

That being said, this error should be benign and don't cause state loss.
I understand, you got some points, but then after refreshing you had 0? Though I haven't experienced it, I can say that the state is stored locally (IndexedDB). An inability to save to this location or clearing this data will be responsible for losing game state.

Edit: I see other people also report this, but I'm unable to reproduce it.
Maybe some day I will find the cause. For now I can advise a mobile version instead, from Google Play Store (if you use Android).