Out of nowhere last Saturday the ServiceDesk Plus stopped Fetching e-mails. Upon looking at the log files I noticed the following: 

[12:52:09:721]|[09-17-2018]|[com.adventnet.servicedesk.common.MailUtilities]|[INFO]|[116]: Check if the username and password are correct|

Although the password didn't change, nor is it set to expire and I do have the possibility to login to the account through office.portal.com with the password provided in the servicedesk. 

It doesn't look related to the certificate issue.