Did you right click > Settings > Local Storage > Unlimited?
If you don't allow enough memory to save the entire world it could cause errors. Also, closing the browser in the middle of saving does that too.
Could that be the issue, or is it something else?
 "Bearbarians"){kind=icon}
 "Mine Blocks"){kind=icon}
 "Fisher-Diver"){kind=icon}
 "Learn to Fly 2"){kind=icon}
 "Omega Box"){kind=icon}
Did you right click > Settings > Local Storage > Unlimited? If you don't allow enough memory to save the entire world it could cause errors. Also, closing the browser in the middle of saving does that too. Could that be the issue, or is it something else?