Problem found (and solved)
A special German character (Umlaut) was, once again, the cause.
In file /languages/german/portal_islands.php line 37 reads:
$ubbt_lang['DELETE_SHOUT'] = "Lösche Shout ID:";
Replacing "ö" with "oe" solved the problem. It seems that this is the text for a hint and this kind of text shouldn't contain special characters (or should be entered in a special way that I don't know).
Using special characters in other texts (e.g. window captions) doesn't seem to be a problem.
