Problem with SVN+Apache/SSL/SSPI and large commits/imports

I am running SVN 1.6.3 over Apache 2.2 on Windows, using the most recent CollabNet Windows builds. I'm using mod_auth_sspi and mod_ssl to secure my connections and perform authorization. On small checkins and imports, none of my users have any problems doing their work. But when importing or checking in a large number of files, users get prompted for their credentials (which the server promptly rejects), followed by the error:

PUT of <path and filename> 
authorization failed: Could not authenticate to server: rejected Basic
challenge (https://<my server name>)

We've tried using both the command line client and TortoiseSVN.

This error seems to be related to the number of files checked in, NOT the aggregate size of content committed. For example, if I try to check in 90 1-byte .txt files, the import will fail as described. It's not always 90 files -- I think it has something to do with the aggregate size of the filenames. I have this nagging feeling that it SVN is trying to jam all of the filenames into a single HTTP request header or something, and Apache is rejecting it somehow.

Not that I can examine what's going on at the HTTP level using Wireshark, since everything is encrypted via HTTPS. Here's my config file, less any comments. Has anyone run into this before? Any suggestions?

ThreadsPerChild 250
MaxRequestsPerChild  0

ServerRoot "C:\Program Files\CollabNet Subversion Server/httpd"

Listen 443

LoadModule actions_module modules/mod_actions.so
LoadModule alias_module modules/mod_alias.so
LoadModule asis_module modules/mod_asis.so
LoadModule auth_basic_module modules/mod_auth_basic.so
LoadModule authn_default_module modules/mod_authn_default.so
LoadModule authn_file_module modules/mod_authn_file.so
LoadModule authz_default_module modules/mod_authz_default.so
LoadModule authz_groupfile_module modules/mod_authz_groupfile.so
LoadModule authz_host_module modules/mod_authz_host.so
LoadModule authz_user_module modules/mod_authz_user.so
LoadModule autoindex_module modules/mod_autoindex.so
LoadModule cgi_module modules/mod_cgi.so
LoadModule dir_module modules/mod_dir.so
LoadModule env_module modules/mod_env.so
LoadModule imagemap_module modules/mod_imagemap.so
LoadModule include_module modules/mod_include.so
LoadModule isapi_module modules/mod_isapi.so
LoadModule log_config_module modules/mod_log_config.so
LoadModule mime_module modules/mod_mime.so
LoadModule proxy_module modules/mod_proxy.so
LoadModule negotiation_module modules/mod_negotiation.so
LoadModule setenvif_module modules/mod_setenvif.so
LoadModule userdir_module modules/mod_userdir.so
LoadModule ssl_module modules/mod_ssl.so
LoadModule dav_module         modules/mod_dav.so
LoadModule dav_svn_module     modules/mod_dav_svn.so
LoadModule sspi_auth_module   modules/mod_auth_sspi.so
LoadModule authz_svn_module   modules/mod_authz_svn.so

ServerAdmin localhost@localdomain

ServerName sourcecontrol.mydomain.com

DocumentRoot "C:\Program Files\CollabNet Subversion Server/httpd/htdocs"

<Directory />
    Options FollowSymLinks
    AllowOverride None
    Order deny,allow
    Allow from all
    Satisfy all
</Directory>

<Directory "C:\Program Files\CollabNet Subversion Server/httpd/htdocs">
    Options Indexes FollowSymLinks
    AllowOverride None
    Order allow,deny
    Allow from all
</Directory>

<IfModule dir_module>
    DirectoryIndex index.html
</IfModule>

<FilesMatch "^\.ht">
    Order allow,deny
    Deny from all
</FilesMatch>

ErrorLog logs/error.log
LogLevel warn

<IfModule log_config_module>
    LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\"" combined
    LogFormat "%h %l %u %t \"%r\" %>s %b" common

    <IfModule logio_module>
      LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\" %I %O" combinedio
    </IfModule>

    CustomLog logs/access.log common
</IfModule>

<IfModule alias_module>  
    ScriptAlias /cgi-bin/ "C:\Program Files\CollabNet Subversion Server/httpd/cgi-bin/"
</IfModule>
<Directory "C:\Program Files\CollabNet Subversion Server/httpd/cgi-bin">
    AllowOverride None
    Options None
    Order allow,deny
    Allow from all
</Directory>

DefaultType text/plain

<IfModule mime_module>
    TypesConfig conf/mime.types
    AddType application/x-compress .Z
    AddType application/x-gzip .gz .tgz
</IfModule>

<IfModule ssl_module>
SSLRandomSeed startup builtin
SSLRandomSeed connect builtin
</IfModule>

<Location /svn>
   DAV svn
   SVNParentPath D:\repository
   AuthName "Subversion repository"
   AuthType SSPI
   SSPIAuth On
   SSPIAuthoritative On
   SSPIDomain NA
   SSPIOfferBasic On
   Require valid-user   

   SVNAutoVersioning on
   ModMimeUsePathInfo on
   AuthzSVNAccessFile "d:/repository/access-file.txt"
</Location>

<VirtualHost *:443 >
    DocumentRoot d:\repository
    SSLEngine On
    SSLCertificateFile conf/ssl.crt/server.crt
    SSLCertificateKeyFile conf/ssl.crt/server.key
</VirtualHost>