Have exact same issue. v5.2.6 on Debian, getting error 500 with Android app.
Have webapi module loaded and get http://<server ip>:8000 returns "{"description": "NetXMS web service API", "version": "5.2.6", "build": "5.2-443-ge9a696f0c2", "apiVersion": 1}", so this looks good.
Have webapi module loaded and get http://<server ip>:8000 returns "{"description": "NetXMS web service API", "version": "5.2.6", "build": "5.2-443-ge9a696f0c2", "apiVersion": 1}", so this looks good.