Is there a common maintenance issue requiring filling a hole in the tail on the A321?
While maintenance records exist at the airline, they're not something that can be accessed by the general public over the internet.  And without those records, it would be guessing to answer what exactly "filled a hole with putty near the tail of the plane" was addressing.

It sounds like the delay might have been to allow the putty to cure, but again, without knowing the exact details, that's as much a guess as anything.

If you know somebody who works in the maintenance department at AA, they might be able to get the info, but again that's not the sort of thing that airlines like being released routinely.

This question may well be closed as being essentially unanswerable, and that's probably the right "answer" in this case.  Without far more specific knowledge, it's not something one can determine from the facts given, even with the World Wide Web available for further research.