I have a problem with my backpack. It was filled with many items and fully upgraded. I put it on my back, and after that, when I pressed the key to open it, it was completely empty—no upgrades, no items.

This isn't the first time I've encountered this issue, both in single-player mode and on multiplayer servers, across multiple versions.

I don't know why this happened, but I want to know if there's a way to recover the many hours of gameplay and the unique items I've lost.

Thank you very much.

potentially the backpack was duplicated and deduplication logic set it to new UUID. you can try using /sbp list /sbp give commands to get your backpack but if it was really duplicated then that's not going to help as another player has already opened "your" backpack and it will be assigned to them in the history of backpacks open most recently.

Anyway if you feel like there's anything to fix here I will need more info to be able to recreate, but this is such a basic thing that if that really was an issue in general I would have hundreds if not thousands of reports like this and yet I don't see them.

Second reporting of this happening.
In my case, I had a diamond-tier backpack (but otherwise no content, colour or upgrades) and put a couple of Twilight Forest loot items in, and stored it in a Iron Chests chest of diamond tier with other content. When taking the backpack out again, it was suddenly completely empty and the TF items were gone.

Third reporting of this issue.
Yesterday I left leftover blocks from construction in a backpack (potentially the same ID, cannot figure out), left it on my hotbar, and logged out. Today I log back in, and all the blocks are gone. I even went as far as checking the NBT of my playerdata, the data of the associated backpack NBT, and checked the same data in backups. It is nowhere, even in backups that should have the items still in.

It looks like this particular backpack doesn't save its NBT or something, but I cannot see or tell why it wouldn't. There is and were never any illegal or corrupt items in it to cause its NBT to break down.

Additional thought that may be incredibly relevant here, this backpack might have been a drop from a loot piñata. Maybe the mod doesn't allow the same UUID to happen multiple times, and maybe I didn't observe duplicate backpacks getting dropped at any point, but the attempt might still have happened and corrupted a UUID in my save. That's the only cause I can think of why this one particular backpack doesn't save correctly.

This at least gives a potential for repro steps, however I haven't been able to repro it yet even with this in mind.

yeah loot pinata duplicates backpacks including the UUID and then if 2 backpacks with the same UUID exist in inventory of a single player or 2 online players one of them will get its UUID changed to prevent item duplication / loss bugs. 1 backpack retains its UUID and thus its inventory though. Backpacks are actually not corrupted or saving incorrectly just have the UUID deduplicated.

Anyway if the backpacks were duplicated this way I am going to just close as it's working as intended. Unless you have any additional info that this is not the case and steps how I can recreate.

That is not what happens. I brought up loot piñata because it would make sense that the backpack could get corrupted because of one.

Never has the backpack been dropped multiple times from the same mob though, I was under the impression you have a mechanism already to account for duplication of that kind and prevent it at least partially.

Whenever a loot piñata drops a backpack, only one backpack is dropped. So this might not be related after all. Then I don't know anything about why it could happen.

I have no way of preventing loot pinata from dropping multiple backpacks so if it's only dropping 1 that must be something on that mod's side.

The issue could also potentially be caused by a crash happening at the time of server saving data.
If that's not the case I will need to understand steps I can use to recreate otherwise there's nothing I can do here. This obviously isn't a common "backpacks sometimes not saving contents" issue otherwise this tracker would be full of those reports. So there's something special going on here.

Are you able to provide steps to recreate or should I just close?

Personally, I've not been able to reproduce it consistently yet. Or even find more factors to determine what causes it. It definitely is still a huge frustration for me that this happens, as well. But I can't speak for the OP on whether to close this issue.

One thing that I found, was that most of the time this happens, I was inserting items into a backpack using the "click item onto backpack item" like a bundle. But unfortunately, I can't say that's how it happened all the time. I also definitely opened the GUI before and inserted normally, only to realise upon reopening immediately afterwards, that the entire batch of insertions was without persistence. So this detail doesn't consistently say anything.

Another thing I found, was that when the issue happened in the GUI of the backpack, it was always empty prior. But, again unfortunately, this wasn't the case when inserting stuff into it like a bundle. I've been inserting stuff into the backpack while clicking items onto its own item, and there was already content in it, but nothing new was added in the end. So nothing consistent in this detail either.

The only thing consistent is the modpack it happens in, but then again, it's the only one I'm playing, so that doesn't say much.

If lag is the cause for this, then try to interact with backpacks in a modpack that keeps hitting the 90%'s of allocated mem usage. Then maybe at some point the mod runs into a tick where it can't fully perform its save, and then cause this issue. But, nobody knows for sure that's the actual cause.