Not sure rn whether this just applies to clerics or to everyone - either way: it's buggy
Something about the block state property not existing on minecraft:lava_cauldron

Fixed in 1.2.2

happens to me too, only happened when i was a cleric class

It's only the cleric, can confirm. Client only crash as well, crash log in pastebin

Okay, so people already found this. I'll post my pastebin as well.
More specifically, I crashed when trying to fill the cauldron with a water bucket. Don't know if it applies to filled cauldrons.

clicking on a cauldron with a right hand, empty or not, crashes the game

I've found I'm able to extend potions still, as long as they're regular potions, not splash or anything. My workaround rn is asking someone else to fill it, or letting the rain fill it. Filling it myself or otherwise rightclicking with anything else will crash my client.

Edit: New info? It seems to always crash when the water level is at stage 1 (lowest water amount). Just tried extending 3 of the same potions in a row on a full cauldron. The first two succeeded, but the last one crashed. My splash potion test earlier also crashed at cauldron stage 1. An interesting bug for sure.

the worst thing about this is, that this repo seems like is using old libraries which are unavailable, so we can't event help fix it, because noone can run the setup script

Not sure rn whether this just applies to clerics or to everyone - either way: it's buggy
Something about the block state property not existing on minecraft:lava_cauldron

Just Clerics. Offering my confirmation, and a crash report
latest (2).log

this is from an empty cauldron, to a full cauldron with a water bucket. Normal function for all classes but Cleric.