I'm trying to open a ID5 file in ID5.5 and I get the same error message you do.
Isn't that lovely. (The power supply died on my Macbook that was running ID5. I checked the file version using that script: yes, it's ID5. ...There's a snowstorm and the place that sells power-cords is out of stock for a couple days. Should I go on?)
I also exported the ID5 files in IDML format since I saw that mentioned. Again, same error!
I have a PC Win7 laptop w ID5.5 on it. Whew, I thought I was rescued! Silly me.
I just want to do my work. Using the same software. Using the standard concept that a newer program can open a file from an older one. ...WHAT IS THE MATTER?
I've restarted my PC several times, restarted ID...
Is it just one file (as in your first paragraph) or have you tried opening others (your third paragraph seems to be saying that you've tried multiple files) with the same result? How did you export the file(s) to IDML if you couldn't open them (which you would need to do to export them as anything)? Have you tried trashing your preferences? To do so hold down Ctrl + Alt + Shift when launching InDesign and respond affirmatively when asked if you want to reset.