can anyone explain me what this means? Is it serious?
This is the same problem that was recently discussed on this list; your filer's memory detected and corrected some bit errors in its RAM in DIMMs J27 and J30. This is not unusual and you will see them from time to time, particularly the more the filer is loaded. Since you were doing 29GB of writes for 3 hours, at nearly an average of 1000 ops/s, it's not surprising that the memory got utilized a lot and a few bits had to be corrected.
The messages are only serious if you get a lot of them in a row or if its consistent or related to another problem or crash. If you want to feel safe, you could take the filer down for a few minutes and reseat those DIMMs just to make sure they have good connections. (If this is a new filer in production, perhaps it's been moved around lately and thus some connections may have gotten loosened up.)
If Netapp says it's a serious problem they can send you replacement memory. However, these sorts of errors are common. Be thankful that your filer has error-detecting error-correcting memory and didn't crash on a memory parity error like the old filers used to. :)
Bruce