Hi,
The error 'XML Parsing Error: no element found (...) Line Number 1, Column 1' means that the sitemap could not be generated. Probably because of the resources restrictions of your hosting and the fact that the hikashop plugin will have to load all the products and categories in memory which is probably too much to handle and it goes over your resources restrictions (probably the memory limit).
The best would be to look first in the PHP error log of your server for the fatal error generated during the creation of the sitemap.
Then, if you have an error, you'll know what is the origin (memory issue, execution time issue, bug, etc).
If there is no fatal error, then try increasing both memory limit and execution time on the server.