java.lang.NoClassDefFoundError: dev/shadowsoffire/apotheosis/adventure/affix/socket/gem/GemItem
i keep getting this error when i try to join single player

I'm assuming you're running the latest Apotheosis on 1.20.1 with one or more of the following mods:

• Passive Skill Tree
• Loot Beams
• Extended Combat Apotheosis Plugin

In the future though, be sure to share more information such as a full latest.log or crash report when reporting issues, it makes it a lot easier for people to figure out what went wrong.