I'm fairly new with Alfresco so I'm still learning the ropes.
My issue initially was that workflow notification emails were not sent out so I modified the Alfresco-global.properties file as follow:
I've set the following to true:
notification.email.siteinvite=true
And I've added the following line according to Email Notifications in workflows :
### E-mail site activity setting ###
activities.feed.notifier.repeatIntervalMins=5
Afterwards I tested to see if emails were sent out but not as of yet. After I restarted the VM to see if that might help, I got a new error when testing the workflow after pressing Approve or Reject:

I since have removed the added lines and undid changes mentioned above. I have also restarted the VM but still end up with this error.
Thanks in advance.