Search thread does not exist. Can't get derived key for
Hello, 

I started to get this error messages constantly

![image](https://user-images.githubusercontent.com/6827668/51693542-39a21580-2010-11e9-9b52-8f6467c92827.png)

Also when I try to get unspent outputs, I get this error . Sometimes it returns unspent outputs, but most of the time I get this 'Search thread does not exist' error

```
sergey@DESKTOP-BGBIKND:/mnt/c/Users/ankar$ curl  -w "\n" -X POST http://monero.node.com/get_unspent_outs -d '{"address": "address", "view_key": "view_key", "amount":"0","mixin":4,"use_dust":false,"dust_threshold":"1000000000"}'
{"reason":"Search thread does not exist.","status":"error"}
```

Ubuntu 17.04

Thank you