Since CMSimple 4.0.3, a new page separator is used, because the previously used seperator ":" has caused problems with links from facebook & Co.
Facebook changes this colon to "URL encoded" in it's links, and the corresponding link leads to nowhere. I don't like facebook & Co., but no CMS should ignore facebook at the moment ...
For new installations of CMSimple, you don't have to think about the delimiter, everything is tuned on the new separator "___" (3 x "_").
Converting of old CMSimple websites to CMSimple 4.x is another thing. A new URI seperator causes new URLs for menulevel2 and menulevel3 sites, and all internal links and incoming deep links are faulty.
Moreover Google indexed pages are lost, with all search results and Google PageRank. Until the new pages are indexed in the search engines and the rankings are as before, can take some time.
If you upgrade an existing website to CMSimple 4.0.3 or higher, you should think twice about, whether working links from facebook & Co. are more important than the old, well-linked and "indexed in the search engines" pages.
If you think, your old pages are more important, you should set the URL separator back to ":" (colon). The corresponding configuration option you will find in the CMS configuration:
URI => Seperator