Error when importing a XAR backup from a subwiki to another on instance running with MySQL

Preconditions: The instance of XWiki must be configured with MySQL Database and an empty subwiki should be already created.

STEPS TO REPRODUCE

1. Login as Admin and create a subwiki by choosing the "XWiki Standard Flavor - Sub Wiki"
2. In the new subwiki, create a page (e.g. Pag1)
3. Go to Administer Wiki -> Content -> Export
4. Click Export
5. Save the backup XAR file locally
6. Go to the empty subwiki
7. Click Drawer -> Administration, upload the XAR file and click Import

EXPECTED RESULTS

All the pages/documents are successfully exported from one subwiki and imported to another (empty) subwiki without any errors.

ACTUAL RESULTS

The importing process is finished with an error:

 Error while importing - 1 Page(s) with error 

and the in the "List of erroneous pages" section is displayed "XWiki.XWikiPreferences" Page (screenshot attached).

The error is encountered only when the instance is running with MySQL Database and is reproducing no matter if the subwiki on which the XAR file is imported was created with the standard flavor or no flavor at all.