Microsoft Office Excel cannot access the file ‘https: //Remote.Contoso.com:987/test/test/test.xls'. There are several possible reasons:
‘The file name or path does not exist.
‘The file is being used by another program.
The workbook you are trying to save has the same name as a currently open workbook.
When you try to access Excel files from SharePoint Document Library Internally (http://companyweb) everything works
If you change Alternate Access mapping from https://Remote.Contoso.com:987 to http://Remote.Contoso.com:987 and remove the SSL Binding from the SharePoint website everything works
You can follow below mentioned blogs for your reference
Introducing the “Add a Trusted Certificate Wizard” in SBS 2008
How to Manually Install Certificates in SBS 2008
Article ID: 2417228 - Last Review: Oct 9, 2012 - Revision: 1