Error: ffmpeg.FS('readFile', 'output.webm') error. Check if the path exists

[G4udy]

Describe the bug
Im interested download some ugoira to HEVC mp4 using FFMEPG using the following command:
-f concat -i c:\PixivToolkitSettings.txt -vcodec libx265 -safe 0 -c copy output.webm
PixivToolkitSettings.txt is empty

When I press Generate Custom, I get this message:
Error: ffmpeg.FS('readFile', 'output.webm') error. Check if the path exists

pd: idk if is a bug or Im doing wrong

Example work from Pixiv
exmple: https://www.pixiv.net/en/artworks/123980254

Screenshots

Browser and Version (please complete the following information):

• Firefox
• Pixiv Toolkit 5.5.3

Settings of extension
{"MangaPageNumberStartWithOne":false,"alwaysPack":false,"animationJsonFormat":1,"askDownloadSavedWork":true,"autoActivateDownloadPanel":false,"disableDownloadsShelf":false,"displayWorkTypeLabel":true,"downloadPackFiles":true,"downloadPanelPosition":"center","downloadPanelStyle":1,"downloadRelativeLocation":null,"downloadSaveAs":false,"downloadTasksWhenDownloadingImages":3,"enableExtTakeOverDownloads":false,"enableExtend":false,"enableExtension":true,"enablePackUgoiraFramesInfo":true,"enablePtkSearch":true,"enableSaveDownloadHistory":1,"enableSaveVisitHistory":true,"enableWhenUnderSeconds":1,"extendDuration":3,"featureKnown":false,"guideShowed":true,"illustrationCreateSubdirectory":0,"illustrationImageRenameFormat":"","illustrationKeepPageNumber":false,"illustrationPageNumberLength":0,"illustrationPageNumberStartWithOne":false,"illustrationRelativeLocation":"","illustrationRenameFormat":"","importantNoticeDisplayed":true,"language":"default","mangaImageRenameFormat":"","mangaPageNumberLength":0,"mangaPagesInChunk":99,"mangaRelativeLocation":"","mangaRenameFormat":"","maxDownloadRecords":10000,"maxHistoryItems":10000,"multipleDownloadsGapTime":150,"notSaveNSFWWorkInHistory":false,"novelIncludeDescription":false,"novelRelativeLocation":"","novelRenameFormat":"","pixivComicImageRenameFormat":"{id}{title}p{pageNum}","pixivComicPageNumberLength":0,"pixivComicPageNumberStartWithOne":false,"pixivComicRelativeLocation":"","pixivComicRenameFormat":"{id}{title}","showPixivOmina":true,"showReloadInPopup":false,"showUpdateChangeLog":true,"statIllustDownloaded":0,"statMangaDownloaded":0,"statNovelDownloaded":0,"statUgoiraDownloaded":36,"ugoiraConvertTool":"ffmpeg","ugoiraCustomFFmpegCommand":"-f concat -i c:\PixivToolkitSettings.txt -vcodec libx265 -safe 0 -c copy output.webm","ugoiraQuanlity":10,"ugoiraRelativeLocation":"","ugoiraRenameFormat":"{id}{author}_{authorId}","version":"5.5.3","visitHistoryType":"list","workCoverSize":1}

Regards,