Hi Takayuki,
It’s a custom theme made by our developer. You’re right it could be a plugin that we perhaps don’t have installed on our other site that is very similar. We have 57 active plugins on our main site with the dysfunctional contact form. The other site with the functioning contact form (https://pro.duri.com/contact-duri/) has 51 active plugins.
If you have the time, is there a way you can find out how to fix it? Our web developer is busy at the moment and a customer pointed it out yesterday to us the page stopped working.
Is there anything else you need to know so I can help you help us? Do you think it’s an issue with the coding? It was working fine before but this has to be a recent issue. We did update all of our office computers to Windows 10 last Friday, i’m not sure if that can be a factor. The last contact form message we got to our email that I can see was February 11.
Thanks Takayuki! Love the quick support and both plugins!