Start smplayer, choose Video/Thumbnail Generator, choose options, click generate. One of two failure modes then occur.
First failure mode: dialog complaining 'The file /tmp/smplayer_preview/00000001.jpg doesn't exist. (It does, created manually). Or it says that file 'can't be loaded'.
Second failure mode: A small dialog with titlebar 'Thutor' appears briefly, followed by the Plasmashell 'oops' showing a mpv crash.
This problem started appearing on five different systems immediately after upgrading to Fedora 33 (from, variously, F30, F31, F32 one version at a time). On all five systems the thumbnail generator worked fine on F30 through F32. It still works on an up-to-date ubuntu 20.04.1 LTS.
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[20-12-19-15:28] 20<Lacrocivious>30 I use the Thumbnail Generator in smplayer with mpv backend extensively. This worked very well until I upgraded five different systems variously from Fedora 30, 31, and 32 to 33. Now it is broken on all five F33 systems but still works on an up-to-date Ubuuntu 20.04.1 LTS system. Is this a known issue? I've not found anything from DDG searches
[20-12-19-15:29] 20<Lacrocivious>30 I learned in #mpv that Fedora does not package mpv at all, so I must have gotten mpv from rpmfusion. Might the package maintainer for mpv for rpmfusion be lurking about?
[20-12-19-15:38] 19<leigh123linux> Lacrocivious: mpv doesn't do thumbnails
[20-12-19-15:40] 20<Lacrocivious>30 leigh123linux: Ah, thanks, that is useful information. Thumbnails are coming then from ffmpeg?
[20-12-19-15:40] 18<28leigh123linux18> Maybe smplayer was broken by my recent update to mpv-0.33.0 update
[20-12-19-15:41] 19<leigh123linux> Lacrocivious: Try installing ffmpegthumbnailer
[20-12-19-15:41] 20<Lacrocivious>30 I attempted ABRT report but eventually got: Retrace server is unable to process package 'mpv-0.33.0-1.fc33.x86_64'
[20-12-19-15:41] 20<Lacrocivious>30 ffmpegthumbnailer is installed
[20-12-19-15:42] 20<Lacrocivious>30 It may or may not have been updated in the (4 to 5 thousand) packages upgraded during F3x to F33
[20-12-19-15:44] 19<leigh123linux> Lacrocivious: What filemanager is failing to show thumbnails
[20-12-19-15:44] 20<Lacrocivious>30 On one sysstem kffmpegthumbnailer is also installed but this appears to make no difference. On the other systems only ffmpegthumbnailer is installed
[20-12-19-15:44] 20<Lacrocivious>30 Dolphin is showing thumbnails fine. It is only generating thumbnails from within smplayer that stopped working with F33
[20-12-19-15:46] 20<Lacrocivious>30 Unfortunately I am not a coder and I am thus uncertain about what tree to be barking up at May very well not be anything to do with mpv
[20-12-19-15:53] 20<Lacrocivious>30 leigh123linux: By the way I am honored to speak with you. I have certainly benefitted from your myriad contributions to several projects
[20-12-19-16:18] 19<leigh123linux> Lacrocivious: I can reproduce the smplayer thumbnail generator issue with mpv, it works fine with mplayer
[20-12-19-16:19] 19<leigh123linux> Lacrocivious: Can you file an issue against smplayer, maybe it needs changes for newer mpv