In YieldLogic.sol#executeYieldSetERC721TokenData(), reference this check:
In this case the user calls YieldStakingBase.sol#Unstake() then the admin triggers executeSetManagerYieldCap() to set the ymData.yieldCap to zero. The user now cant call YieldStakingBase.sol#repay() because the transaction will revert to the above check.
The NFT of the user is locked. the only way to unlock it is by the admin, he needs to re-set ymData.yieldCap > 0. However, the user will pay the interest from his yield when he waiting for the admin to re-set the value. To fix this just move this check inside the first IF block:
That way, the user can call repay() in this scenario.   
