The topic is old but I spent a lot of time to solve the problem that appeared in me.
Check the /etc/resolv.conf file.
If localhost (127.0.0.1) is not in the first place then change that.
If you are already using dynamic addition to resolv.conf, the default option on server providers, then make sure that the first place is 127.0.0.1. You can achieve this by editing the files in /etc/resolvconf/resolv.conf.d/ in the head file add entries:
nameserver ::1
nameserver 127.0.0.1
I have spent a lot of time to come to the conclusion that if you have a foreign server entered and it is slow or with restrictions, the whole plesk panel works slowly and even can fail to work.