Agatha's Soul Cauldron Phyrexian Mana Bug
When exiling a creature with a Phyrexian mana activated ability (Skrelv or Rona, for example) under Agatha's Soul Cauldron, there is a bug where we cannot activate those abilities at all — it shows the total mana cost as 6 colorless in the case of Rona, but gives us no option to actually pay to transform her.

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Elec Boy#97212 commented
All of your creatures’ activated abilities, not just the ones given to them by the cauldron, are affected — as long as they cost phyrexian mana.
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infernalspectre#83223 commented
This happened to me as well and has lost me multiple games as I am no longer able to keep Skrelv available while tapping out my mana and using my life as a resource.
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tactics#01060 commented
+1, here's a screenshot with it happening with Skrelv.
I think this is due to the "You may spend mana as though it were mana of any color to activate abilities of creatures you control." line on the card. The choice should be between 1 generic, 1 W, or 2 life, but instead it's just 1 generic.