Dave B

Thank you, that was it. I have no idea how it got turned off – I did not even know that gamerule existed.

I thought I'd add an additional comment: the real bug here is that, although the launcher knows that the backup failed (an error message is issued after all), the launcher still marks the world as though the backup succeeded. This means that if one dutifully follows the instructions to make a backup but, for whatever reason, misses the error message and clicks Play then the launcher no longer issues the warning and launches the game. This permanently destroys the prior version of the world and the user has no valid backup.

The launcher should not reset the warning unless a backup is successfully made.

+ since worlds are by default created on ones C: drive – along with just about everything – it is very useful to be able to move the very large minecraft worlds to another drive having more available space

+ other archive programs such as 7z have no problem "doing the right thing' – an archive is created which contains the files which are referenced by the symbolic link

+ the minecraft launcher warns and encourages one to make a backup but then fails

+ after failing to make the backup, the launcher >with no further warning< will launch the game which effectively updates the world to the newer version of minecraft

I followed your instructions and have the log url: https://paste.ubuntu.com/p/pRf6CybvdH/

Here are the exact steps I used to create the problem:

Minecraft launcher:
Using an installation with '20w22a' active, create a new world
and call it '20w22a'
[All default options]

Play that world briefly for a minute or two
[F3 shows 20w22a (20w22a/vanilla/snapshot)]

Quit/save the game & launcher

Using windows explorer, move saves/20w22a to E:/worlds
Result is E:/worlds/20w22a

In windows using cmd.exe "as administrator":
c:
cd \users\imdave\appdata\roaming\.minecraft\saves
mklink /d 20w22a e:\worlds\20w22a\
Symbolic link created for 20w22a <<===>> e:\worlds\20w22a\

Start minecraft launcher

In Minecraft launcher:
Click on installations
select installation '20w22a'
click "..." and then "edit"
change version to "snapshot 1.16-pre1"
click "save"

From "Play", with installation "20w22a" active, click "Play"
After the game starts, choose single player, select world "20w22a"
I see the warning "Don't forget to backup this world before you load it in this snapshot"
I click "edit"
I click "make backup"
I get warning/error message in top right corner: "! Backup failed, C:Users\imdave\Appdata\.. " (remainder truncated)

I then quit/saved the game & launcher and have provided you with the launcher.log

Note: I also tried with 1.16pre2 and got the same results.
It appears to me that when you try to create the ZIP backup file, you are not followng the symlink – the ZIP contains exactly one file of zero length and is the name of the world.

This has the significant potential of ruining days or weeks of work by rendering existing farms totally broken. I really, really don't want things that i have built being destroyed by the game.

Still broken 1.15 pre4
See the additional screenshot.

An even more simple setup. Two droppers facing each other. Dropper on the right contained a single item (dirt). Flipping the switch does nothing for the nearer pair of droppers. The further pair of droppers (offset by 3 blocks from the south edge of the chunk) works as expected: the dirt block moved to the left dropper.

Very frustrating for someone trying to learn redstone.

Still fails to work on 1.15 pre1. The left droppers do not xfer their item after receiving a redstone signal. Makes no difference with different delays on the repeater next to the observer, I even powered the dropper directly with a lever – still nothing. The dropper fails to respond.

I just encountered this today. 1.13.1, Java 64-bit 10.0.2, Win7, 3GHz core i7, 4GB memory allocated to Java. I've covered the bubble columns (12 total, soul sand @ 36, bubble column to 93 encased in stone bricks. FPS begins to improve when I am around 7 blocks away. Closer than that, my frame rate drops to 1-2.