Use the safeApprove() method in setLiquidationPair() to set the allowance.
The safeApprove() method requires the current allowance to be 0 for the setting to succeed. If the liquidationPair_ has been used before, this is the second time it is used and there are residual allowances, it may not be possible to set it.
I suggest to clear the allowance to 0 before setting.
