Remote server returned an error

I have been experiencing issues with sending out emails and now saving invoices. I get the below error message when saving invoices and I am only able to send emails to one of my clients. I use all the default settings and have not been using any port forwarding or SMTPs of any sort. Have anyone had the same issues or can shed some light on a fix?

Running Manager 15.3.47 on OSX Yosemite 10.10.5.

System.Net.WebException: The remote server returned an error: (500) Internal Server Error.
  at System.Net.HttpWebRequest.CheckFinalStatus (System.Net.WebAsyncResult result) [0x00000] in <filename unknown>:0 
  at System.Net.HttpWebRequest.SetResponseData (System.Net.WebConnectionData data) [0x00000] in <filename unknown>:0

Try to upgrade to the latest version from and see whether problem persists.