This message is pretty suspect:
LogOnline: Warning: STEAM: Failed to initialize Steam, this could be due to a Steam server and client running on the same machine. Try running with -NOSTEAM on the cmdline to disable.
Assuming you are not running 2 Unreals both connected to Steam Online Subsystem, the other possibility is that there are multiple processes listening/sending on one of the Steam ports. I had a problem where some weird Steam Home Beta thing was listening port on 27015, in addition to Steam OSS.
I would suggest that next time this happens, do a “netstat” on all the ports found here. I believe that warning also happens if there are multiple processes messing with one of the ports Steam uses, in addition to Unreal using that port