The Money Laundering Act prevents German Missions abroad to assist with signature certifications and identity verifications in all bank matters, for example when opening a bank account or taking a mortgage. Requests for the certification of a signature must be refused.
Please consult the bank you have been in contact with for alternatives.
An exception is made for blocked accounts in relation to visa applications for study in Germany. German Missions may still certify the applicants signature to open a blocked account if this is the most reliable way for them to prove their financial means. See here