I found the answer to this question on the Subversion mailing list. The problem is that the Python 2.7 SVN bindings provided by Trac were built using an older version of Apache dll, namely libapr-1.dll, and reference a function called _apr_time_ansi_put @ 8. In later versions of Apache, including VisualSVN 2.1.5, libapr-1.dll is newer, and the function has been renamed to "_apr_time_ansi_put @ 12".
To fix this, I downloaded the old Apache bindings from http://subversion.tigris.org/files/documents/15/47914/svn-win32-1.6.6.zip , renamed VisualSVN \ bin \ libapr-1.dll to libapr -1.dll.bak, copied the old libapr-1.dll to the VisualSVN \ bin folder and restarted the service. No more problems, python from svn import core is working fine, and the validate-extensions.py hook file is working correctly.
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