"ERROR Connection Timeout" on gatt.connect()

I am using the MDBT42Q with software version 2v04 and the console on Serial1.

The problem I have is that gatt.connect() only rarely succeeds connecting to either of 2 BLE peripheral devices, in more than 9 out of 10 times it throws "ERROR Connection Timeout". I am using very basic code:

NRF.requestDevice({ filters: [{ name: 'devicename'}] })
.then(function(device){
    console.log("found - trying to connect");
    return device.gatt.connect();
})
.then(function() {
  console.log("Done!");
})
.catch(function(err) {
  console.log("ERROR", err);
});

I am trying this with 2 BLE devices: A BLE sensor and the nRF app on Android set as an advertiser (discoverable, connectable).

I am using a 2nd Android phone with the nRF app as BLE Central to scan and connect (and then disconnect) to either of these 2 BLE devices, which works reliably. As far as I know no other BLE central device is trying to connect to the devices.

Given that it works from the nRF app and there are very few parameters to tweak the connection establishment in the MDBT42Q I am not sure what to try next. Any help is appreciated.

Thanks