I reverted back to the 2.3.6.1 way and it works now.

As an additional task: Please inspect the tables mlf2_logincontrol and mlf2_settings with phpMyAdmin. Which value has the setting temp_block_ip_after_repeated_failed_logins? Are there data rows in the table mlf2_logincontrol and, if yes, with which values?

Mlf2_settings temp_block_ip_after_repeated_failed_logins had a value of 30.
mlf2_logincontrol I would have to check later.

Thanks for your help.