The items in my backpack disappear

Only one data backpack's item competely disappear,but class level,items in the bank didn't,same as other data.
Is it my problem or a bug or something?
Please help,I don't want to train another data for quest equipment.

 

What do you mean by "data" exactly?
And what do you mean by "backpack"? Is it your inventory? Your emerald or ingredient pouch?
If you are saying that one of your classes' items completely disappeared from your inventory, then that might have been caused by repeated death.
But I can't give any more answers until you clear up what you mean exactly.

 

bug, cannot revert, send bug report if you know why

in a server someone said that their entire inventory got wiped out in one class, probably the case here

 

It's just like what you said

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I'll go bug reports after getting home