The session start error usually means that an extra character (usually a space) has been added at the beginning or end of a file. If it is at the beginning, it will be easy to find since the first character should always be <. If it is at the end, you will need to move your cursor to the very end of the file and backspace until you reach the > character. The difficult part in fixing this problem is in finding the correct file. Always start with the last one you edited, the one in the url or the one mentioned in the error. Those are the most likely.
Posted in: OsCommerce