Unfortunately, Mac Excel 2011 is quite buggy, and running macros sometimes results in crashes. We have tried to address this issue, but due to the inherent instability of the platform, we can not fully resolve it. Microsoft will hopefully address the issue in future patches. Meanwhile, we suggest you save the file often, or if possible, use the tool on a PC, as it's very stable on Windows and faster too. Another option is to use Windows Excel on Mac running Parallels Desktop, VMware Fusion or Boot Camp.
Often, the errors appear randomly and are not related to the type of report you run. In these cases, restarting Excel usually helps.