First things first: Thank you léonie!
Well, I used iCloud Photos for a while, but then, I reverted it to “Off”. So I’m not using it right now, but I used to do it.
I’ve never used “optimize storage” though.
All of the “problematic” videos were recorded like eight years ago, with an iPhone 5S, and later synced to Photos on a MacBook Pro (locally).
Storage is not an issue today, nor was it in the past. I’m used to have way more storage capacity than I actually need, so... not any problem there.
Network? I’ve discarded that, because I’ve tried over (several) wifi networks and over my -decently fast- LTE mobile network, without success.
About the codecs/wrappers... It’d be really disrespectful from Apple, to just drop support for any given codec without heaving us, users a REMARKABLE heads up about our valuable content! I don’t really know what codecs are those videos, but I assume they are H.264 coded. Since I don’t have access to them anymore, I can’t even make sure about that.
what is really strange, is the fact that I CAN PLAY BACK certain videos from that same phone! In some cases, they’re even older than the ones I can’t reproduce now.
Anyway... I appreciate your kind response, Léonie!