This problem can easily be fixed when bouncing the database, but the problem was that this production database could not be stopped. I had to find another way to solve this.
Could it be they are using a wrong version of plsql developer ? I couldn't find out if they were using a wrong version, so upgrading plsql developer seems useless....
I found another work around for this problem.
Maybe it has something to do with the "debug information" mode. To verify this, go to Tools > Preferences > Debugger tab page, disable the "Add debug information when compiling" option, and retry the compilation.
The developers tried to compile the package again and it worked...