Help Galaxy S5 Bluetooth - Connect to BLE devices

[zxmon21]

Hi,
first time I'm asking a question here, so bear with me.

I own a Samsung Galaxy S5 (SM-G900F, Android 6.0.1, rooted), which I want to connect to a heart rate monitor (bluetooth low energy = BLE, chest belt) and a bike sensor (also BLE).
I put brand-new batteries in both sensors, and they show up fine in S5 Bluetooth scans ("Bike Combo sensor", "BLE Chest strap"). I also made sure to make the S5 visible to other devices.
When I select one of the sensors, the S5 will try "Pairing..." for a couple of seconds, then I get an error message "Can't communicate with Bike Combo sensor" (or chest strap, depending on which I'm pairing to).

That sucks. Please help.

For diagnostic purposes, here's some extra information:
I have paired my S5 with lots of other devices (speakers, computers, other phones) without trouble. I was not connected to any other Bluetooth devices when I tried connecting to the sensors.
My wife's Motorola X Style and iPad connect to the sensors just fine.
I revived a second old S5 (same android, also rooted) that I have lying around the house. Both S5 have the same trouble connecting to the sensors.

HELP!

 

[zxmon21]

So... After trying for weeks to PAIR my BLE heart rate monitor, I figured it out today:

BLE devices SHOULD NOT BE PAIRED!

Read e.g. https://support.runkeeper.com/hc/en-us/articles/203496577-How-To-Pair-Your-Heart-Rate-Monitor-
2B. If you are using a BTLE HRM, do not pair the HRM to your phone and proceed to step 3
Instead, the app (e.g. Runkeeper) has a way to connect to the sensor (little heart symbol on the start screen)...

So it's the app that has to find the BLE device, no pairing...