The File creation problem (line 473) is due to mis configuration of webserver directory permissions.
There is no way of 'fixing' this from within Drupal, you just need to give Drupal write permissions to the webserver root.
Alternatevly use Sansui's fix, create the file rss.xml in your website root and give it write permissions.

I have re-written the file handling and messaging part for the new 'quick release' (minor) .

Pavol Kutaj messaged me that foreign language punctuation was filtered out from stock item description, this was fixed in the minor release.
He also had problems with product image links, but that is due to misconfiguration or non standard image configuration on his part .
His third comment was to ask if it is possible to use URL cloaking (hide the stock item true url path) this is not possible unfortunately.

Pavol, it is not necessary to use another sitemap creation module, this one does the job.