Yea, sometimes the browsers don't like the savefile size. I currently don't understand the underlying cause, the file should be way to small to cause any issue (0.1 mb at most) and when testing it on my own setup with the same browsers, I am unable to replicate this bug.
For future reference: What browser and operating system were this?
Edit: It appears max save size might be limited to 5-10 mb for the entire itch.io domain on some browsers. So other games on itch might count against the limit