Redirect to 404 Error page after Updating Post and Page using SiteOrigin Buidler

Hello,

I have been using Flash Theme (themegrill.com), Free Version since last Month.
Everything are working fine but from last week after the update on SiteOrigin Builder, certainly issue occored on my site Backend.
The issue is,
When I Update the Page or Post using Site Origin Page Builder, it will redirect to the url (https://ktmonlinemart.com/wp-admin/post.php) “Oops! That page can’t be found.” page

While create new Post and Page, it works fine but while editing and saving same post it show error.

Please help me on this issue. I have been stocked on this issue trying to figureout for more than 4 days. But still No Luck for me.

Could you please let us know what is the exact issue is???

Have a Great Day!!!

Best Regards,
Sujan

The page I need help with: [log in to see the link]