Possible incompatibility with scoreboard plugins
Software: Paper 1.16.5
Plugins: AnimatedScoreboard(or any similar scoreboard plugin), ModelEngine.
Other plugins don't affect the result.
The issue: Every time player joins a server with enabled scoreboard many errors are thrown in console. This issue doesn't persist in higher versions (1.17+).
Log from test server: https://pastebin.com/6JeCVWht
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information