Bitcoin c-lightning node on Synology cannot connect to bitcoind


I’m trying to setup a bitcoin lightning node on my NAS (Synology), following this tutorial: https://bitcoindev.network/running-a-lightning-node-on-synology-nas/

Unfortunately I get the following error (log):

lightningd: WARNING: default network changing in 2020: please set network=testnet in config!
bcli: 
Could not connect to bitcoind using bitcoin-cli. Is bitcoind running?
    
Make sure you have bitcoind running and that bitcoin-cli is able to connect to bitcoind.

You can verify that your Bitcoin Core installation is ready for use by running:
    
        $ bitcoin-cli -testnet echo 'hello world'   

2020-06-30T19:45:20.256Z INFO plugin-bcli: Killing plugin: Plugin exited before completing handshake.
The Bitcoin backend died.
lightningd: WARNING: default network changing in 2020: please set network=testnet in config!

My lightning.conf contains the following:

alias=myalias
network=bitcoin
bitcoin-rpcuser=myusername
bitcoin-rpcpassword=mypassword
bitcoin-rpcconnect=127.0.0.1
bitcoin-rpcport=18332
log-level=io

I’ve tried several parameters for bitcoin-rpcconnect (internal ip, external ip of my bitcoin node including the right ports).

Since my bitcoin node is running on mainnet I try to setup the lightning node for mainnet as well.

So does anyone knows what I’m doing wrong?