[2024] Local server unable to initialize, can't start new solo game.

The logfile indicates that files that are necessary for Eco's operation are no longer present. That is typically caused by overzealous antivirus software deleting things silently. In any case the installation is corrupted and needs to be repaired, at the same time you need to figure out what is removing things.

Steam wouldn't even notice that, given AV can just block access to files or quarantine it, without them actually being gone. The issue is definitely a local one with something meddling with Eco. It may be necessary to remove AV or reach out to their support to figure out how to actually disable it, tons of AV software behave like viruses themselves and when you disable them don't actually stop all security measures.