The internet address " https://......" is not valid

I was not sure if this was an administration issue or a general issue, please feel free to move it to the appropriate forum.

Our SharePoint site (https://) external users have been facing this error for a long time and we are not sure how to tackle it. It says"The internet address (path name) is not valid". This error happens when the external user is trying to open the document from a document library, tries to edit using MS Word (happens to other PPT's as well) or tries to download a copy. We are within the intranet and none of these issues occur so we don't exactly know where to start.

Also if someone has any other ideas to open documents from the SharePoint library while this gets sorted would be helpful.