So you’re working on an incredibly long form string process, and after submitting, you hit the dreaded: HTTP Error 404.15 – Not Found: The request filtering module is configured to deny a request where the query string is too long error. Well, don’t fret, because this is a very easy problem to fix.
Step 1: Open IIS Manager
Step 2: Roles > Web Server (IIS) > Internet Information Services
Step 3: Select your web server on the right side
Step 4: Double-Click Request Filtering under the IIS section
Step 5: Click Edit Feature Settings on the right side menu
Step 6: Increase the Maximum URL length (Bytes) and Maximum query string (Bytes) fields. 1024 = 1KB, so I have my web server set to 8192 Bytes (8KB)
Step 7: Click OK and Restart your web server
These steps fixed my issue without a problem, and hopefully they will for you too!