I have same issue , on windows 10 bedrock edition since 1.16, all worlds where i have visited nether in 1.14.60 version can't be opened.
Seems to be ok with worlds where i did not enter nether
I'm having same issue with some of my worlds (i'm on bedrock edition windows 10)
From what i guess, it's only in worlds where i went to nether in previous version (1.14.60)
In worlds were i did not visit nether , it's ok
Would be happy to be able load them again, especially one which was in creative mode were i made / tried a lot of things with redstone and would took weeks to create them again
I have same problem on some of my worlds , in Windows 10 bedrock edition since 1.16 , and not fixed in any version.
I think this problem occurs only in worlds where i have entered the nether before 1.16 version either in creative or in survival. I don't have problem with worlds in which i did not go to nether before this version