When you want to optimize a world from an older version which would be opened up in safe mode in 1.16 because the vanilla datapack is disabled and not all tags are supplied, it sends you straight back to the title screen. The world then disappears from your singleplayer menu for a couple of seconds and later returns with the message "Locked by another running instance of Minecraft".
When trying to open the world a warning appears in the output log. While the world is not yet visible in your singleplayer menu another warning appears when opening the singleplayer menu. See attached output_log_warn.txt file.
I've tried this with worlds made for versions 1.14, 1.14.4 and 1.15.
How to reproduce:
Open an older version of Minecraft, create a world and disable the vanilla datapack
Create (or copy) a different datapack into the world which does not contain all vanilla tags. As long as the game recognizes it as a datapack.
Try to open (not optimize) the world in 1.16 RC 1
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I am unable to reproduce this, updating a world where the vanilla data pack was disabled to 1.16 re-enables the vanilla datapack (that might actually be a separate bug), it did not lock up or anything like that, please provide more details about the world in question.
Just tried to reproduce this myself again, this time simply opening the world worked fine. Optimizing the world however still gives me the same result I had before, where it sends me straight back to the title screen and locking the world. I'll edit the bug report. @Galaxy_2Alex could you try again?
The vanilla datapack is a core part of the game, and should not be disabled. But that error msg IS weird, maybe they should fix that?