While importing from transwiki or upload a file I get the following warnings:
Warning: XMLReader::open(): Unable to open source data in \includes\Import.php on line 65
Warning: XMLReader::read(): Load Data before trying to read in \includes\Import.php on line 494
Special:Import shows: "Import failed: Expected <mediawiki> tag, got "
The return value of XMLReader::open is not checked in WikiImporter (in this case it is false) and therefore there is a first read on the xml file (which also returns false). After the first read is a check which expected a <mediawiki>, but nothing is read and therefore no tag is there. So error handling can be improved here.
The question is why open the source data fails. Special:Version shows "5.5.9 (apache2handler)" on a windows machine. I found some old imports on my wiki but there where all under 5.4 or 5.3.
I have added some displays to verify that the http request against dewiki is okay and gives xml back.
There is a tmpfile() call, maybe there needs to be some settings, but than mediawiki should give better messages.