For ASP.NET 1.1, this is still due to someone posting more than 1000 form fields, but the setting must be changed in the registry rather than a config file. It should be added as a DWORD named MaxHttpCollectionKeys in the registry under
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\ASP.NET\1.1.4322.0
for 32-bit editions of Windows, and
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\ASP.NET\1.1.4322.0
for 64-bit editions of Windows.