Red errors during startup usually come from broken XML patches or outdated mods. Identify the mod and fix the XML first.
output_log.txt and search for the first red error.Free tool: The free 7 Days to Die Log Analyzer does this search for you: drop output_log.txt in and it flags failed XML patches with the responsible mod, malformed XML with the line and position, and missing blocks or items left behind by a removed mod - all in your browser, nothing uploaded.