Pls help in this error ... Method Not Allowed POST /irr?_s...

Hi

This error has cropped up all of a sudden while I have been making minor changes to the site
www.uxfin.com

"Method Not Allowed POST /irr?_s..."

this comes when you click on the last button [Add Data Lines] at the bottom of the page.

Pls advice on what it is and how to solve

regards
Sanjay

Ok ... seems its narrowed to to cases where I click the button without entering any info in the immediately two preceeding date fields - so will probably have to initialise that

regards