The other party the channel is connected to can close it at any time. To get around this, open a channel with a more stable node or switch wallets if your current one won't let you pick a node yourself. (also complain to the wallet provider about those funds not coming back to you if you can't pick what node to connect to and it doesn't just select a random one for you).
Thank you very much, and sorry for the delay — I wasn't expecting such a quick reply.
I have been using BTC with Electrum for many years, after moving all funds off the exchanges, but in a rather low-key manner (mostly shopping needs). Lightning is very new for me, so please bear with me, I'm not at all familiar with some of its workings.
Following the indications of an instructor who seemed experienced and reliable, I modified Electrum's config file in order to connect to one node only (instead of various nodes selected at random). He recommended a couple of them. I opted for 'blockstream.info:700'. That same instructor recommended opening the channel with ACINQ. So far it had been going OK — another channel opened with Electrum trampoline got closed on me without my understanding the reason.
Do I understand correctly that you would suggest I reinstate the use of multiple nodes? As far as the wallet itself is concerned, I only use Electrum and I wouldn't know what else to use. Also, Electrum only gives me three options when opening a channel (incidentally, 'hodlister.co' was also recommended by the instructor, but only as a single node, not to open a LN channel with).
I'm not sure how I should complain to Electrum (the wallet provider) — anyway, the fees lost were really minimal, single-digit euro cents.
Last, is the 'Boltz backend', which Electrum designates as "powering submarine swaps" in any way related to all this?
Anyway, whether it's Boltz, ACINQ or blockstream.info that's responsible for this, what could be the reason for it or they doing it?