I think I fixed the problem with the attachment of text files in the forum.
I realised that I actually need the full log, even the output texts when smplayer closes. To do so, go to preferences -> advanced -> logs and check the option "Save smplayer log to a file". Now close smplayer, open it again, play a file, close smplayer. And then attach the smplayer_log.txt that you'll find on the smplayer configuration folder.
BTW, before running smplayer to create the log, be sure the power settings are configured properly, as I want to see if smplayer is saving/restoring those settings correctly.