I had same issue. I knew i had my castle move left if opponent put me in check, but instead it claimed i lost to check mate and didnt allow me to castle queenside.
Not able to castle - Chess960 - iOS app and web interface/server disagree on rules

I had same issue. I knew i had my castle move left if opponent put me in check, but instead it claimed i lost to check mate and didnt allow me to castle queenside.
You can not castle out of or through check.

Hi,
I have the same issue in Chess 960. When I try to castle in the iOS app it tells 'Game Update Failed – Skip/Retry'. When I try to castle in the web app, it doesn't do it at all. In Analysis mode during the game, castling is an option. After the game ended and I checked it in the moves Analysis, castling when I planned was the best move, but was not doable due to the bug.
Can you please fix this?

Hi,
I have the same issue in Chess 960. When I try to castle in the iOS app it tells 'Game Update Failed – Skip/Retry'. When I try to castle in the web app, it doesn't do it at all. In Analysis mode during the game, castling is an option. After the game ended and I checked it in the moves Analysis, castling when I planned was the best move, but was not doable due to the bug.
Can you please fix this?
There had been a bug on the iOS app where 960 game incorrectly created the FEN for the position without the castling rights. So, if the game was started with a bugged version of the app, you won't be able to castle since the game setup shows that castling is not possible.

There had been a bug on the iOS app where 960 game incorrectly created the FEN for the position without the castling rights. So, if the game was started with a bugged version of the app, you won't be able to castle since the game setup shows that castling is not possible.
Hi Martin, thank you for the explanation. I tried new 960 game and castling works now!
In this game: https://www.chess.com/game/daily/293369964 I am not able to castle queenside as black.
In the mobile app on iOS I am able to castle correctly when using the analyze feature. I can also make the move but when I do it tells me he game could not be updated. If I try to castle from the web interface it doesn't even recognize that it's a valid move.
From everything I know the iOS app is correct and I should be able to castle. But can someone double check me?
Either way there is a bug here since the two interfaces disagree with what the possible moves are.
iOS app version v3.9.11 (3) on iPadOS 14.2.1 and iOS 14.2.1 and 14.3