Check the version number on your Lightroom. It should say 7.0.1 release in about and "Lightroom Classic version: 7.0.1 [ 1142117 ]" in Help->System Info. You should see a lady with purple glasses in the splash screen. If you see leaves in the splash screen you are running Lightroom CC which is a completely different and new program. Adobe decided to confuse everyone by using the name of an existing program for something completely new and incompatible in many ways with the old product known by the same name. The Lightroom you know is now called "Adobe Photoshop Lightroom Classic CC". You can update your Lightroom from the creative cloud app but make sure to choose the classic version, not the CC version!
Thank you for the reply, but I don't understand. I upgraded to the CC version earlier this year from LR 6 stand-alone. I am running the PS/LR photographer subscription bundle. I don't understand the difference between Lightroom Classic and Lightroom CC. This is what I see in my System Info :
Lightroom version: CC 2015.13 [ 1141928 ]
License: Creative Cloud
Operating system: Mac OS 10
Version: 10.10 
I've been processing work for this photographer all year with no problems, so I don't know what would have suddenly caused an issue. I updated through the app this morning when I got the error, but updating didn't fix the problem. Should I uninstall and re-install?
Yeah you are running the old version. The update to this version is called "Lightroom Classic CC" and you can find it in your creative cloud app. Just install it from there and you will be able to open the catalog files from your photographer who has already updated his Lightroom to the latest version. Note that in creative cloud app you will see an option called "Lightroom CC" which is not the same thing and will not open your photographer's catalog files! Adobe just wanted to confuse everybody clearly.
thank you - i wasn't aware adobe had split the app into two different versions. i'm sure there's an email sitting in my "promotions" folder that i haven't seen. that's why i was so confused. totally makes sense now. and i've also confirmed that clients have upgraded to "classic" without telling me - so that is indeed the problem. appreciate the help! thank you!