Win admin? Yes should be. Where can I see this for sure?
Command line: net user [your-user-name]
There should be something like "local group-membership: administrators"
Write protected upload folder. Can this be some security thing with XAMPP? Any suggestions how I can check this?
No, I don't think so.
In the upload folder is remains (a folder named imBlogging) from imBlogging module that is now uninstalled. Does this bring any clues?
No, but you can delete it in case you are not using imBloggin anymore.
Regarding the php test file I was talking about : Save the following code as anything.php within your ICMS root directory and check whether it works. It should create a file named testFile.txt within the thumbs folder. If there is such a file after executing the php there is no writing protection issue.
$myFile = XOOPS_ROOT_PATH."/uploads/thumbs/testFile.txt";
$fh = fopen($myFile, 'w') or die("can't open file");
$stringData = "testn";