Try spending more coins.
That was the problem. Thanks again!
Type in lightning-cli listpeers on your other node, find that channel and compare "our_channel_reserve_satoshis" and "their_channel_reserve_satoshis" values.
I've opened a channel from both Electrum's node and Core's node to 1ml dot com's node, because apparently I can't make those two connect together, so 1ml routes my transactions. With a
listpeers from Core Lightning I see:
"their_channel_reserve_satoshis": 15000
"our_channel_reserve_satoshis": 15000
I sent another 20,000 sats, which means I've sent 30,101 over all, plus a few in fees. Now my receiving capacity is 10,110 sats. What's the explanation behind that? The number would make sense if the reserve was 20,000 sats. (I haven't checked the reserve from Electrum's console as I don't know how to do it atm, but it shouldn't be different; it's the same Lightning node)