At work and don't have Windows 10 or FirebirdSql here, so I can't test my theory.  But I suspect its permissions related.

There is a new feature on the properties dialog for files in Windows 10.  Its a checkbox to unblock a file.  Right click on the security.fdb, select Properties and see if that option is there.

No "unblock" check box is shown for security2.fdb. I forgot to mention previously that I changed the security properties to Allow: Full Control, Modify and Write for All Applications Packages, SYSTEM and Administrators.

But ...

On a more radical solution, try installing your firebirdsql to a folder NOT in Program Files. 

Yes, this solves it!

So it is definitely a permissions thing, but I haven't been able to find out what yet.

Thanks again, at least I am up and running now.