Whenever you launch Minecraft, it tells you you must change your skin, as it is from a content pack not available in multiplayer games. This, however, happens even when your skin is perfectly fine for multiplayer games.

Temporary fix is to just change your skin and change it back.

How to reproduce:

1. Close Minecraft
2. Open Minecraft
3. Join Realm/External Server/ Friend's Server
4. Issue should come up
   If the issue still isn't seen, try again but with step one close it via task manager