The error occurs when you try and expand the public folders in exchange 2003 system manager. First thing to check is if HTTP keep-alives are enabled on the site that is hosting exadmin.
1. Right click on the Virtual Web site that hosts the ‘exadmin’ virtual
directory and click Properties.
2. On the Web Site tab, make sure to check the “HTTP Keep-alives Enabled”
3. Click OK.
4. Run iisreset in the CMD prompt.
If the issue still exists follow the steps below.
1. Launch ADSI Edit by running adsiedit.msc.
2. In the left side pane expand the Configuration container.
3. Next expand CN=Configuration
4. Then CN=Services
5. CN=Microsoft Exchange
6. CN=<your organization name here>
7. CN=Administrative Groups
8. CN=First Administrative Group <or it may be your original Exchange 2000
13. Right Click on CN=Exadmin and choose Properties.
14. In the Properties dialog box you will see 2 drop-down lists. drop down
the top list and select “Both”. Drop down the second list and scroll down
to the attribute “msExchSecureBindings” and double click on it.
15. If this attribute is set to 443 or any other value really, click the
443 value to select it and click the “Remove” button. Then click “Apply”
and then “OK”
16. Close out of ADSI Edit, close and reopen Exchange System Manager and
test Public Folder access again.