Running DNN 5.02.03 with Windows Server Web, .net 3.5 and SQL Server 2008
Basically all user account changes fail (like changing user first or last name) with this message in the log (snipped):
System.Xml.XmlException: Invalid character in the given encoding. Line 121, position 15. at System.Xml.XmlTextReaderImpl.Throw(Exception e) at System.Xml.XmlTextReaderImpl.Throw(String res, String arg) at 
Upon investigating, it looks like the problem stems from a call to aspnet_Membership_GetUserByName where the parameter @CurrentTimeUTC is given as for instance 2010-05-05 07:59:55.6870000. SQL chokes on this as a DateTime format, as there are too many characters - if the trailing zeroes are removed it works (valid string for DateTime is YYYY-MM-DD hh:mm:ss.nnn). From DNN code, this DateTime string comes from DateTime.UTCNow. 

Any clues on why this happens, and/or what to do about it?