"Content Type Edit/Add" gives Whitescreen, no errors loged, what to do?
OK, resolved. Thanks for your help,  Triskelion - thanks to you, I understand  xampp better. 
There were two things
1) xampp giving no error messages. After looking in vain for error messages, I upgraded to xampp 1.8.0, and when I tried to get to the page that caused problems, I got a full error message. 

 Additional uncaught exception thrown while handling exception.Original ODOException: SQLSTATE[HY000]: General error: 2006 MySQL server has gone away: SELECT * FROM {system} WHERE type = &amp;#039;theme&amp;#039; OR (type = &amp;#039;module&amp;#039; AND status = 1) ORDER BY weight ASC, name ASC; Array ( ) in system_list() (line 165 of /opt/lampp/htdocs/twotrees/includes/module.inc). Additional 
PDOException: SQLSTATE[HY000]: General error: 2006 MySQL server has gone away: SELECT ff.* FROM {filter_format} ff WHERE (status = :db_condition_placeholder_0) ORDER BY weight ASC; Array ( [:db_condition_placeholder_0] =&amp;gt; 1 ) in filter_formats() (line 409 of /opt/lampp/htdocs/twotrees/modules/filter/filter.module).
Uncaught exception thrown in session handler.
PDOException: SQLSTATE[HY000]: General error: 2006 MySQL server has gone away in _drupal_session_write() (line 209 of /opt/lampp/htdocs/twotrees/includes/session.inc).


Still, no messages in error_log. Where should I look for them?

2) I searched through the web, and found similar problems for example [http://drupal.org/node/1542432]. It turned out this is a XAMPP mysql memomry limit problem. I changed the value 'max_allowed_packet' to 100M in 'my.cnf'. 

My site works now. I was really scared!