Hi,
Once you locate the detailed error message in the logs, it should provide you with more details about what may be the problem. If it doesn't make sense, I'd recommend searching for that error message in the archives of this dspace-tech list (as you may find an immediate solution from someone else who had that same issue). If you cannot find a solution, you can post the error message to this email thread and someone on this list will try to help.
Tim