Spell Queller with Teferi, Time Raveler (BUG)
In summary, player was not able to cast creature spell after Spell Queller left the battlefield - while Teferi, Time Raveler planeswalker is also in play
Player casts Kraul Harpooner, Opponent exiles it while on stack with Spell Queller... During 1st main phase of the following turn, player casts Hero's Downfall - targeting Spell Queller - and it resolves; Spell Queller leaves the battlefield
This resulted with Spell Queller's trigger going on the stack, however, there was no mechanism available to cast the freed up Kraul Harpooner; it shows up in its own temporary pop-up window but everthing is greyed out except for a "No" button...
Upon consulting with the rulings on Gatherer website, player should have been able to cast a creature spell at sorcery speed during player's 1st main phase (even with the presence of Teferi, TIme Raveler), per the fourth ruling listed for Spell Queller on Magic Gatherer website.
For this issue, please submit at: https://mtgo-support.wizards.com/hc/en-us/requests/new and select 'Reimbursement Request / Report An Issue'.