Sharepoint 2013 apm iapp , document editing not working

Hello, I have integrated APM in front of SHarepoint 2013 through iApp template, login and SSO is working okay. We can download document , upload it etc. What is not working is Web app preview or editing document .

 

Error popup just says "Sorry, there was a problem and we can't open this document. If this happens again try opening the document in Microsoft Word."

 

Could you point me which logs i should look for (i'm a little new to this parts). Thank you