Hi!
Update: I also have a clean Joomla test installation with only CB installed. I noticed the following behavior on this test site AND my normal installation:
1. Field params, Registration = no: Tools->Check Community Builder Database shows no errors
2. Set Fied params to Registration = yes: Tools->Check Community Builder Database shows the following error:
Core CB database structure differences:
Table #__comprofiler_fields Rows fieldid = 52 : Field registration = 1 instead of 0
3. Fix DB -> Field params is again set to Registration = no
Frank