NBT null error on Servers / Command issues

Howdy (:

I am currently trying to do a simple mod that lets you execute certain commands that give items to players or change certain variables.

It worked finde until the last changes I made. I only added commands for sending messages to players and changing a number variable for a player.

So whenever i try it on the integrated server environment it kinda works fine, but there are two main issues:

1. When trying to execute any command for multiple players (@a) it doesn't work and spits out a huge error in the server console, so it does not even work for some reason. I tried adding the "For each" block but that doesn't work either.
2. The more severe issue is an error players get when joining after I installed my mod on the server. It says this:
   

I would highly appreciate any help on this because I honestly do not know what to do. The error is super weird and has no log entries. Players just instantly disconnect after joining. I also looked into the recent additions I made in my mod but I can't find anything suspicious that could trigger such a thing.

I just know MCreator seems to mess up with the commands I added, but no clue how to fix it.

Thank you in advance!