BackupAgent: 'Not enough storage space available' message when upgrading a TB account

Problem:

When upgrading a TB account to an account type higher than 1000 GB, you receive the message 'Not enough storage space available', even though there is enough storage space available to increase the account size.

Cause:

This is caused by a bug which sees the new account type as an extra TB.

Solution:

Set the account temporarily back to GB, providing there is enough allocated storage space to mirror the temporary account size.