Have you tried importing into Flash Builder? If you don't have a copy, there is a free trial.
Use Project > Flash Catalyst > Import to bring the project into Flash Builder.
You can then drill down the project into the main.mxml file. You can then post the MXML tags around line 217 to this thread. We might be able to find the issue.
Thanks for the tip. I do have the whole Master Collection, so opened my project in Flash Builder, and checked the warning. I don't know anything about Flash Builder since I work in design, but assumed the yellow warning triangle was referring to the line 217, which read:
I went on to delete those two lines and saved the project under a different name - and was able to open it in Flash Catalyst again! What a relief!
But how come Flash Catalyst does this kind of thing? Like I said, I was able to publish the project no problem, but then when went to open it again, the error message appeared. So really the user did nothing in between and FC comes up with an error.
There were also a whole buch of other alerts when I opened the project in Flash Builder (with the blue i signs), but I assume that I just ignore those?