This is a very old bug that Niantic hasn't been able to fix yet. A workaround seems to be to deploy a Beacon on the portal, which unfreezes it and allows you to destroy it.
I've had this issue too. It's not frequent but it is annoying. I don't think it's fair to suggest a solution that requires a payment. I.e. battle beacons require a payment.
Comments
This is a very old bug that Niantic hasn't been able to fix yet. A workaround seems to be to deploy a Beacon on the portal, which unfreezes it and allows you to destroy it.
I've had this issue too. It's not frequent but it is annoying. I don't think it's fair to suggest a solution that requires a payment. I.e. battle beacons require a payment.