a) rebuild the index of dokuwiki
b) deleted dokuwiki-cash
c) deleted browser cash
That all was not sufficient. In a data table there are still "orphane-red-dotted-line-links" which point to the pages/sites in (1). What I did to solve this was to click on one of these links and then to create the page, save it, edit it, delete everything and save again.
That seems quite complex. Isn't there a less complicate way. Because my method would also have worked without having done a), b) and c).