Hi, 

this has been happening for a year now and I thought it would sort it's self out but apparently not! 
I casually logged on one day to play on my favourite world I've been working on with some friends for months and then I log off thinking nothing about it, I come back then next day and it says..."The world is not fully synced or has been corrupted.try playing the world on a console where it is in a good state and try migrating it again". 
Ive tried making a copy but when I click on the pencil (edit symbol) it comes up with the same message. Ive also tried hard restarting and clearing local saved game data. You name it I've tried it all. I though I wouldn't give up and I'd contact u guys for help!