Magento 2 API PUT Method & Coupon Input

I need some help about my Magento 2 site. * Please do not offer something without read carefully. The coupon inbox is not working on my Magento 2.2.4 site's payment page. When i checked with Chrome's developer tool; i saw this error: Uncaught SyntaxError: Unexpected token < in JSON at position 0 at [login to view URL] (<anonymous>) at [login to view URL] ([login to view URL]) at Object.<anonymous> ([login to view URL]) at fire ([login to view URL]) at [login to view URL] [as rejectWith] ([login to view URL]) at done ([login to view URL]) at [login to view URL] ([login to view URL]) - there is no wrong on file permissions and chmods; i'm sure. I re-install magento 2.2.4 and theme again to make a fresh test; but the problem is the same. (the theme owners are saying 'they are tested several times; and everything is ok at their site; and also i saw their site is working too) I'm not sure if its a server config problem or not (i checked all topics on Stackoverflow and Google; but couldn't get the solution) By the way; i saw this 404 error for API PUT method on chrome console at the same page: PUT [login to view URL] 404 () In Network tab; i see this error ([login to view URL] is my site): Request URL: [login to view URL] Request Method: PUT Status Code: 404 Remote Address: [login to view URL] Referrer Policy: no-referrer-when-downgrade By the way; i also saw this error in Apache logs (but not sure if its about this problem or not) 2018/06/08 20:53:29 [error] 3436#3436: *28844 upstream prematurely closed connection while reading response header from upstream, client: [login to view URL], server: [login to view URL], request: "GET /customer/section/load/sections=cart&update_section_id=false&_=1528491190921 HTTP/1.1", upstream: "[login to view URL] sections=cart&update_section_id=false&_=1528491190921", host: "[login to view URL]", referrer: "[login to view URL]" the coupon input is working in cart page page (i mean; there is no error in coupon process) could someone help me with $20-30 budget?