DotNetNuke 4.5 introduced FCKEditor as new default editor, and I really love this editor, that has much more features and IMO is easier to use than the previous one. To follow web standards, it replaces non-ASCII characters like German Umlauts with its html equivalent, e.g. "ä" instead of "Ä". Unfortunately, DNN portal search is not able to handle these HTML sequences, it simply stops when reaching the ampersand character and makes this important feature nearly useless for use in several languages.
When investigating this feature, I found a setting of the editor, that controls the HTML replacement, called FCKConfig.ProcessHTMLEntities. To modify the setting, you need to edit your settings file (specified in web.config, by default /providers/HTMLEditorProviders/FCK/Custom/fckConfig.js). Create a backup of the file, open it with a text editor and alter line 55 to
FCKConfig.ProcessHTMLEntities = false;
All subsequent edited text will be saved without the special replacements, if you edit previously edited text, all HTML character sequences will be replaced by the non-ASCII character. If you need to replace a large amount of entries, e.g. in TextHTML modules, I prepared an SQL script, that will do these modifications for you, you can download it from here. As with any mass changes for your database, please make sure, to backup it before applying the script!