Multisite to single site

Dear community

I created a website on a WP multisite installation but need to revert back to single site. I’m not very techy. So…

1. I am with godaddy
2. Website in question (www.x.com) sits in a multisite installation (currently running WP3.5.1)
2. I set up a new domain (www.y.com). Installed WP3.8 and uploaded the same theme files. (I thought if this works can easily duplicate it back to original domain later.)
3. I ran an export xml (tools/export) of original website (www.x.com) size 21MB
4. I ran an import of that xml file (tools/import) at new website location – chose ‘upload file and import’
5. Then got the next screen and checked ‘Download and import file attachments’
6. The got internal server error message –
The server encountered an internal error or misconfiguration and was unable to complete your request. Please contact the server administrator and inform them of the time the error occurred, and the actions you performed just before this error.
More information about this error may be available in the server error log.

Stuck now!

Would be so lovely if some clever person could help me?
Regards
Leighflo