Hello,
Are you sure that the user- has sufficient privilege?
https://support.f5.com/kb/en-us/products/big-ip_apm/manuals/product/apm-authentication-single-sign-on-11-5-0/2.html
(APM must fetch password policies, for example, if you select the Prompt user to change password before expiration option in an AD Query action.) If you do not provide Admin account information in this configuration, APM uses the user account to fetch information. This works if the user account has sufficient privilege...
please check account privillege.. and try again.
Regards