My guess is that it has to do with encoding of Greek charaters. But there is nothing I can do from my side. I consider this one a bug and "urge" you to please fix it.
i got the same issue here as Yannis.
As soon as i turn the Networks Main Language to German the "latest activitiy" feed becomes invalid.
Some feed readers and feedvalidator.org tell me that it has somthing to do with german Umlauts.
Heres the feedvalidator message:
line 23, column 33: Undefined named entity: uuml (10 occurrences)
Is there a solution to get the feed working in German.??..otherwise Ning would become useless to me