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There shouldn't be cached information in the GUI client after restarting it.
If you drive to preferences, are you able to "check connection" with no issues? Are you configuring your client entering the server name (or IP)?
Could you send to our support email your client log? C:\Users\<user>\AppData\Local\plastic4\logs\plastic,debug.log.txt
If you re-enter the line in your client host file, doesn't it help?