Problem details:
Yesterday, I was using 'rename' to remove the [no video] tag of the beatmaps downloaded from new.ppy.sh (that was the time when any beatmap which doesn't have a video has that tag when downloading). But I only replace '[no-video]', not ' [no-video]' to empty string, so there is still a space left in the folder name.
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When I open osu! and F5, there are error dialogs appeared, the folder of the beatmaps which has a trailing space at the end of the folder's name can't be processed and disappear from the beatmaps list.
Video or screenshot showing the problem:
osu! version: Stable 20181231.2 (latest)