Help request

Problem
upgraded to spigot 1.21 unfortunately we're getting a failure when trying to open it. please see the attcahed screenhot in game,

What I have tried
we've tried mulitple time to uninstall and re-install. also clear player data and try again and it doesnt work!

What's the output of /backpack version?

same result....
we've tried .4 nd .3 versions

thanks in advance

Please share the output. So that i know exactly what your server config you are running.