BOINC Manager 7.0.64 Windows 64bit uses an incorrect URL when attempting to add BAM as an account manager. Old posts here indicate that an incorrect URL in this dialog is a recurrent problem.

The URL 7.0.64 uses is HTTPS://bam.boincstats.com/

The correct URL is HTTP://bam.boincstats.com/
(Just remove the s from HTTPS and click Next)

This is just an informational note for anyone else who runs into the cannot use BAM as account manager error.

I think the problem is not that the URL is incorrect. The problem is that the 7.0.64 Windows Client uses an outdated Certificate Store so it can not verify the SSL connection to BAM.

You should upgrade the Client to a newer version instead of falling back to a non-SSL connection.