When you try to access excel files remotely from Sharepoint Document Library you get message
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
One of the reasons could be Incorrect Certificate in IIS
Import the correct Certificate and Install it in IIS
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