Quick answer: Read the log, SteamCMD validate, check Java, free port 16261, raise RAM.
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Frequently asked questions (FAQ)
Zomboid starts then dies?
Console/panel log: often Java, missing files after SteamCMD, or port 16261 in use. Validate app_update, free the port.
Out of memory?
Upgrade RAM, keep -Xmx below RAM minus 2 GB.