Hello Philipp,
Thank you for posting question on Microsoft Windows Forum!
Based on the provided error message as well as the SSPR_0014 error probably indicates that because Self-Service Password Reset (SSPR) was not configured beforehand, the automated system has no "proof" it is you. Unfortunately, you can not reset your own password without recovery information, and with no other admin accounts available, you might need to contact Microsoft Support directly or Microsoft's Data Protection Team to manually verify your identity and restore access.
First of all, it is worth checking if the account is a Microsoft account (personal) or organizational (work/school). If it is a personal Microsoft account, you can try to use the Microsoft account recovery form. On the other hand, if it is organizational (Azure AD/Entra ID), you will need tenant-level recovery or contact your IT department.
Otherwise, try to contact Microsoft Support directly for initiating a support request https://azure.microsoft.com/en-us/support/. Microsoft provides a way to recover access when the only global admin is locked out. You might need to prove ownership of the tenant (billing information, domain verification, etc.). This is the only way to regain access if no other admins exist.
Hope the provided information is helpful! If it is. It is appreciated to consider clicking "Accept Answer". Should you have more questions, feel free to leave a message.