I have built a SP-hosted app that uses REST API calls to get data from a list in the Host Web.
The request (from fiddler) is as simple as this:
When Host Web site regional settings are Locale are in English, the requests works as expected. But when I change the Local to Greek or some other languages AND it is an on-Premises SharePoint installation , I get Internal Error 500. Changing back to English and it's OK again.
It always works when the exact same app runs on O365 SharePoint no matter what Regional Settings I use.
tutor_tasks list has nothing really special, two User fields and a text field.