So, look like your hostname is bad.
Modified your hostname in your VPS panel (virtualizor/SolusVM/others) with format 'subdom.domain.com'.
For example, this forum hosting in VPS with:
> hostname -f; hostname -s; cat /etc/hosts
dev.potissima.com
dev
127.0.0.1 localhost.localdomain localhost
### begin - add by Kloxo-MR
46.105.49.194 dev.potissima.com dev
### end - add by Kloxo-MR
So, you can run 'hostname -f; hostname -s; cat /etc/hosts' to know about your hostname.