I believe this may be a duplicate of MC-177445 but the reason for the crash there is unknown so it's possible there are two separate issues here. Either way, it fails to mention the reason for the crash and no one has come forth with an interpretation fo the crash report as of yet so I've reported it here.
Running the command:
/give @p leather_leggings{AttributeModifiers:[{AttributeName:"generic.armor",Name:"generic.armor",Amount:1,Operation:0,UUIDLeast:172265,UUIDMost:296356,Slot:"legs"}]} 1
then attempting to look at the item will immediately crash your client. It seems that this is the case with every item with an attribute value, I would assume this is due to the most recent change to how attributes work. This can be found under the Attributes header of [this page|https://www.minecraft.net/en-us/article/minecraft-snapshot-20w14a].
Linked issues
Comments 4
The problem here seems to be the missing UUID tag (instead of UUIDLeast and UUIDMost), see the dev comment on MC-177085.
@Marcono1234, yes you might be right. I'm so far unable to tell what the proper format is for this, Mr Bok has not given an example of something that would work. It seems this is a duplicate of MC-177085 though.
The proper format is an int array of 4 values. You can use this website to convert between them: https://www.soltoder.com/mc-uuid-converter/
I should clarify that this issue was not present in the more recent snapshots but I'm yet to test for the 20w13's.