This sort of thing has happened to me a few times now, I had a response in my hand and it wouldn't let me cast it in response to a planeswalker activation... Opponent had a Tergrid out and was activating Lilliana to make me discard I had a bounce spell in my hand I wanted to bounce Tergrid in response to avoid them getting what I was going to discard. I had enough mana to cast it and usually even if I couldn't pay for it you could try and it would ask you to pay for it, and your only option would be to click the cancel button it wouldn't let me even drag it out of my hand. like I said this has happened in previous cases as well. What ever is causing this needs to be fixed. Also I tried to report this bug and got redirected away from the form automatically, this is frustrating and feels really rude. Who ever is designing your website needs to not **** off your customers especially when they are coming to report a bug. At this point I'm fuming and I wouldn't be if your website worked as expected.

I have 5 untapped snow lands and untapped sculptor of winter
I cast old growth troll
Opponent casts reject... game asks if this spell resolves, i click resolve
Instead of asking me to pay 3, my spell just gets countered.

The game does not check to see how many mana can actually be used; it just checks number of tapped and untapped lands.

Please fix this!!