To resolve an external inconsistency, you have to resubmit or modify the nomination against the counterparty with which you have a discrepancy.
A BRP can still correct an external inconsistency after the gate closure until D+1 14h30. This means that it can indeed be corrected even after the quarter-hour has taken place.
If the external inconsistency is not resolved after D+1 14h30, a penalty will be applied. The penalty will be the difference in nominated electricity, multiplied by the imbalance price. (Compare this to the happy flow in which the imbalance price is only applied to the difference between the real electricity offtake & injection vs. the nominated offtake & injection)
An external inconsistency can be in the day-ahead (DA) nominations or in the intraday nominations. It is important to resolve it in the right process. e-Nominations will not make any assumptions about which type of nomination a BRP submits solely based on the time of submission. If a BRP submits a nomination via the day-ahead tab after 14h30 (gate closure for day-ahead nominations), it will interpret it as a rectification of DA nominations.
Note: a nomination in DA that has a matching state from the counterparty in (i.e. a nomination for which the status is 'Balance OK' or 'Zero WFC' (waiting for confirmation)) cannot be modified after gate closure.