"SaveGameStateToFileTask()" throws Assertion errors on changing the Game State when variables of type "Item" are Serialized in GameManager

How to reproduce:
1. Open the user-attached project “Test”
2. Open the “Main Scene” scene and enter the Play Mode
3. Click one time in the Game view to enter the game menu
4. Click on any hexagon with the picture
5. Observe the results

Expected results: The Game State changes without errors
Actual results: The Game State changes with hundreds of Assertion errors

Reproducible with: 2021.3.36f1
Not reproducible with: 2022.3.22f1
Can’t test with: 2023.2.15f1, 6000.0.0b13 (Unfixable compilation errors)

Reproducible on: Windows 11 Pro
Not reproducible on: No other environment tested

Notes:
-The messages don’t affect the functionality
-The messages are not spammed when “[NonSerialized]” is added to all public variables of type “Item” in GameManager
-Can’t find the fixed version due to complexity in upgrading and downgrading the project

Full error:
Assertion failed on expression: 'CurrentThreadIsMainThread()'
UnityEngine.JsonUtility:ToJson (object)
GameManager:SaveGamestateToFileTask () (at Assets/Scripts/GameManager.cs:2134)
GameManager:<SaveGamestate>b__203_0 () (at Assets/Scripts/GameManager.cs:2074)
System.Threading._ThreadPoolWaitCallback:PerformWaitCallback ()