Is your ServerOnly.json named ServerGrid.ServerOnly.json and stored in the same dir as your ServerGrid.json?
[2018.12.25-01.56.44:678][ 0]FDatabaseRedisShared::LoadDatabaseConnectionInfo() Failed To to load or parse entries from ServerGrid.ServerOnly.json
It looks like it either can't load the file, or can't load the settings from it. Double check that you've verified the IP of the redis server as well.
{
"Name": "TribeDB",
"URL": "127.0.0.1", <- Verify
"Port": 6379,
"Password": "YourMagicPassword" <- Verify
},