Seemingly random disconnect after around 1 minute

I am using a NRF52 Devkit with S140 to connect to a NRF51 running S110. 

Initially this works as expected and I can connect, discover services read, write etc...

However after around 1 minute the connection is dropped. This always happens. 

The NRF51 (peripheral) reports that the reason was 0x0016 (BLE_HCI_LOCAL_HOST_TERMINATED_CONNECTION), and the NRF52 (central) reports 0x0013 (BLE_HCI_REMOTE_USER_TERMINATED_CONNECTION). So it is the NRF51 that is disconnecting, if I understand correctly. 

There are no calls to disconnect in the NRF51-code however, so I don't understand why this happens. If I am connected using BLE on my PC or my Phone I don't get disconnected. This only happens on the NRF52 using S140. 

Sniffing with wireshark did not reveal anything. Communication looks normal (I do writes and notifications every second between the devices) up until the NRF51 sends a LL_TERMINATE_IND and the connection shuts down.

Any help is greatly appriciated.