Have you tried running Ant with the -X option to see if the mxmlc task provides additional debug output? It looks like that task runs an external command and it screwed up the command syntax.
Since this task is not part of Ant, but an extension written by Adobe, you might have to ask Adobe about it. (Or do you know if there is source code available for this task?)
Deleting is very picky. If you tell it to include or exclude specific files, then it deletes only files. If you know which directory contains the *.cache file, you can specify that instead. For example, assume all of the *.cache files are in a directory named temp. Then the delete would look like:
(By the way, your last sentence doesn't match the delete command you gave, so I assumed that the sentence was correct and the delete command contained a typo)
If you are new to Ant and plan to use it for a while, I really recommend you get a book such as Ant in Action. It is really good about explaining all of the intricacies and contains a lot of helpful example projects.