In my experience, this type of issue with the Excel add-in is due to something having been updated with a patch or update. Most often antivirus software is the culprit, but a Microsoft Office patch/update could also be at fault.
Check the history on your machine to see what updates has occurred since you last knew it was working. If you use a corporate workstation machine, check with your IT department to see if anything was patched over the weekend.
If antivirus was updated, could try to remove last patch/update or you might have to go and edit its settings to stop any blocking or preventing the outbound communication with Quality Center.
As an example, I dealt with an issue with McAfee DLP (Data Loss Prevention) which with a patch turned off some ports, one of which was used by QC communication. Removing the patch stopped the issue.