This may as well be some sort of mod incompatibility issue (probably with magnesium as it's a new mod in our modpack) and also only one of the players on the server is experiencing it but anyways, i couldnt find anything similar in existing issues.

First crash happened as mentioned in the title, after the player had placed and broke a torch.
Second crash happened when the same player tried to rejoin the server.
If needed i can test the same location with another account, but from what i understand this is enough for the beginning.

Thank you in advance.

You do not have the most up-to-date mods. Please update before reporting any issues.

This issue has been fixed.