Hi rwhitehead and welcome to the forum.
The message often has network changes or computer re-organisation/upgrade as a root cause. Sometimes just other software updates, especially windows, or simply some new system security policies.
If you are opening the repport or model via a shortcut something may have changed to make the link obsolete.
If you are 'browsing' to the file in question and selecting it before getting the message it would rather suggest some sort of security constraint that alloes you to see the file but not open it.
You may need to talk to your IT people to see if anything has changed or if any relevant upgrades have been applied to your systems.
It it is your system and you are not on a network you may already know the answer .... although that is not always the case with windows