Hi there glimmrmoon,
It sounds like there's an issue syncing music videos to your iPod classic with iTunes on your Mac, and you're wondering if it's related to FairPlay. Do you see an alert message in iTunes when the issue occurs? The article below provides an overview of FairPlay digital rights management.
iTunes: Frequently asked questions about viewing and syncing videos - Apple Atbalsts
Videos purchased from the iTunes Store have FairPlay digital rights management embedded in the files and are in a format that can be readily synced to a video-capable mobile device. Just like with music you purchase from the iTunes Store, you own the videos you purchase. You can authorize up to five computers to play purchased videos, and you can sync the video to as many mobile devices as you like. A single device can contain purchased content from up to five accounts; iTunes will not allow you to add data from a sixth account.
The iPod classic User Guide, which is linked below, will walk you through the process of syncing music videos to your iPod classic.
https://manuals.info.apple.com/MANUALS/1000/MA1195/en_US/iPod_classic_160GB_User _Guide.pdf
Syncing music from selected playlists, artists, and genres to iPod classic
You can set iTunes to sync selected playlists, artists, and genres to iPod classic if the music in your iTunes library doesn’t all fit on iPod classic. Only the music in the playlists, artists, and genres you select is synced to iPod classic.
Set iTunes to sync music from selected playlists, artists, and genres to iPod classic:
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1 In iTunes, select iPod classic in the device list and click Music.
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2 Select “Sync music” and then choose “Selected playlists, artists, albums, and genres.”
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3 Select the playlists, artists, albums, or genres you want.
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4 To include music videos, select “Include music videos.”
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5 To set iTunes to automatically fill any remaining space on iPod classic, select “Automatically fill free space with songs.”
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6 Click Apply.
Note: If “Sync only checked songs and videos” is selected in the Summary pane, iTunes syncs
only items that are checked.
Ciao.