After some further experimentation, and testing with known, confirmed recipes, the cauldron appears to be broken.
We can put valid ingredients into the cauldron. There is no summon warding active, no conjurations already present. Different PCs have tried the same known, valid recipe (illithid, giants) and it will not work. The biological ingredient gets consumed, and the catalyst stays, and we get the summon failed message. This result normally happens if you already have a summon or conjuration out and there isn't room for another.