When you get a message saying, "Invalid Session ID" when signing into ILLiad, it is showing because the Logon Page submit button text does not match what is in the WebFormValues table. Here is how you fix this issue:
You should now be able to sign in to ILLiad.
If the "Logon" form looks correct in Customization Manager, but the button on the site still says "Login", editing the HTML may be the solution.
Look for something like this:
input id="submitILLiad" class="btn" type="submit" name="SubmitButton" value="Login to ILLiad" size="16" aria-label="Login to ILLiad
You'll want it to end up like this:
input id="submitILLiad" class="btn" type="submit" name="SubmitButton" value="Logon to ILLiad" size="16" aria-label="Logon to ILLiad