Do you have access to any of the failing devices? If so, you could check the device console with adb logcat to see the specific error that's occurring.
The issue might be related to your certificates. Double check that your server is set up properly -- if the certificate has an intermediate CA, make sure that's installed as well. Some certificates aren't well supported by mobile devices, so make sure your certificate comes from a trusted CA who says that their certificates will work on mobile devices.
thanks for your reply. Unfortunatly I dont have access to any of these devices up to now. I already checked the certificate(geotrust), because i could remember that the certificate chain error sometimes prevent the app from being online. But no luck there, tried three ssl certificate checks and all of these were positive. Its an wildcard certificate and the API runs on a subdomain. Can this be a Problem? Or do you have any other idea?
Without access to failing devices, you're probably not going to be able to solve the problem. With access to those devices, you could see what's going on specifically to know for sure whether or not the certificate is at fault or if something else is going on. Without access, we're all just guessing.
As to the wildcard cert, well, I suppose anything's possible, but I'd be surprised if this was the issue.
You might try creating emulators with various configurations to see if you can replicate the issue locally. You can install GenyMotion (https://www.genymotion.com/) to do that.