Thank you for your help.
Searching for
http_host I just found two calls of the variable $_SERVER["HTTP_HOST"], but not the definition of that value.
Finally in the file system I stumbled about the file
/assets/cache/siteHostnames.php containing the line
define('MODX_SITE_HOSTNAMES', '.....');Here I could correct the mistake.
Additionally I had to delete the files named
docid_1.pageCache.php a.s.o. - otherwise all the internal links would fail.
[ed. note: ottogal last edited this post 8 years, 4 months ago.]