Hi Jorge,
As I understand it, one of your customers has been transitioned from FOPE to EOP, now you cannot login to FOPE as before. If there is any misunderstanding, please feel free to correct me.
Based on my experience, if the Office 365 tenant has been upgraded from FOPE to EOP, the features in FOPE have been transferred to EOP. We cannot login in to FOPE anymore. But if you are a global administrator of the tenant, you can sign in Exchange admin center (EAC) to access EOP. Could you check if you are global administrator of the tenant, and if you can access to EOP? Otherwise, you need to use the complex admin roles to meet the requirement.
Moreover, if you are using the standalone Exchange Online Protection Plan (EOP), we have a specific channel for standalone EOP. I recommend you contact our EOP support for more professional support. They specialize in that information. Thanks for your understanding. Here is the link: http://technet.microsoft.com/en-us/library/jj723132(v=exchg.150).aspx
Best Regards,
Anna Shi