Flash doesn't recognize classes upon exporting

Hi,
First of all, new here :stuck_out_tongue:

I was asked to update some information on a certain project. The weird thing is that whenever I try to export the movie from Flash (I work with CS3 Professional), I get errors. Before even changing anything. All of them are reporting MovieClip not being recognized (“The class… must subclass flash.display.MovieClip since it is linked to a library symbol of that type” (and a few SimpleButtons too). There is a total of 57 errors. All the classes import and extend MovieClip.

Now I’m pretty sure this doesn’t have anything to do with the code itself, since this project was already published and tested working as it is. I tried adding Class paths in the preferences menu but this didn’t seem to help.

Any suggestions are welcome.
Thanks in advanced!