Sometimes if you use several languages as : Spanish, French, chineese, Russian, Dutch, Germany or language with special character you can get in your RSS feed this : “An invalid character was found in text content”.
How you can solve this problem ?
1) If you have problem only with RSS feed
Go to Sttings / Reading and at field : “Encoding for pages and feeds ” put : iso-8859-1
The problem will be solved !
2) IF you have collation problem and your SQL database has wrong collation
There is plugin which “It is designed to “fight” the broken UTF8 characters on your WordPress powered site.” With this plugin you can change all characters into UTF-8.