This bug occurs when any player rides any Mount (donkey, llama, horse etc.). When riding the mount, if they try to open their inventory or the mounts inventory, it won't open.
If they then get off of the Mount, the player is unable to open any other inventory (their own, chests, other mount inventories, furnaces etc.).
This can be fixed by quitting out of the world, then going back into it.
Thank you for your report!
However, this issue is a Duplicate of MCPE-63196.
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