Maybe not xml but rather CSS not loaded.
The reason could be that the webserver configuration is not fully correct, i.e. rewrite module of Apache not installed or not correctly configured (the option AllowOverride needs to be set to the value All for the install folder of Elgg for the rewrite rules to fully work). This would have to be set up in the server's Apache config.
In some cases you might have to change the RewriteBase option's value in Elgg's .htaccess file if you have installed Elgg in a subdirectory and Apache uses aliases.
The data directory must be fully accessible (read/write) by the webserver process so it might be necessary to adjust the file permissions if the directory.
If the error still persists I probably have forgotten some other possible reasons for the issue and you might come back and reposrt that the problem is not solved yet.
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