Hi found a solution, although I do not really like it.
If I used the "get" method instead of the "post" it is working !!!
Place this code in the form to have the "Create Folder" Button
<form method='get'>
{.if|{.get|can upload.}|
Folder name: <input type='text' name='fldname' maxlength="25" size="25" >
<input type='submit' name='createfolder' value='Create Folder' >
/if.}
</form>
and add this code to generate the folder
[special:begin]
{.if|{.and| {.get|can upload.} | {.urlvar|createfolder.} .}|{:
{.mkdir|%folder-resource%\{.urlvar|fldname.}.}
:}/if.}
Vars in this case are visible in the address bar, but if the user is not logged in and does not have upload rights, the folder is not created, so it's not that dangerous
But there is still an annoying bug: if you create a folder and afterwards you want to delete it, the vars are not cleaned (as "delete" uses "post" and not "get") and the folder is created again!
If you navigate somewhere else, the get is cleaned (no vars in the address bar) and you can delete the folder