CC2015 cannot convert newer version file error message
Explanation: I have to go back to CC2015 for a single workflow that uses InTools Side Heads plugin which has not been updated to the latest two versions of ID. Every time I go back to do the work in CC2015, I have to convert files that are being used by numerous co-workers in CC2018.
Lately, whenever I open CC2015 and try to open one of those newer files, I've been getting the message that
"InDesign could not open the file because it has been created in a newer version . .. . To convert and open this document you need to sign in to Creative Cloud. Please go to Help -> Sign In and enter your Adobe ID. Restart InDesign, then try opening the file again."
The problem is that I AM signed in. When I go the Help menu, it even shows me signed in.
I used to be able to do this, so I don't understand why I'm now getting this message. I have logged out, logged in, and restarted 2015, and closed all the other Adobe apps. I'm aware that I can go back to 2018 and export IDML files, but that was an extra step that I've not had to do in the past. Not sure why this error is blocking my workflow.
Any ideas?