SOLVED: can't find existing collections / add movies in them

[siro]

I checked in the file log_20240812.log, and the action to add to a collection doesn't create any new line in the log file.
The only lines that appeared since I made an attempt are those ones:

[2024-08-12 23:37:13.677 +02:00] [INF] [77] Emby.Server.Implementations.Session.SessionWebSocketListener: Sending ForceKeepAlive message to 1 inactive WebSockets.
[2024-08-12 23:40:25.678 +02:00] [INF] [52] Emby.Server.Implementations.Session.SessionWebSocketListener: Sending ForceKeepAlive message to 1 inactive WebSockets.
[2024-08-12 23:40:37.681 +02:00] [INF] [82] Emby.Server.Implementations.Session.SessionWebSocketListener: Sending ForceKeepAlive message to 1 inactive WebSockets.