The import of a XAR package in a new subwiki fails sometimes in Jetty

STEPS TO REPRODUCE

1. Login as Admin
2. Install some extensions, create some pages
3. Go to Administer Wiki > Content > Export
4. Click 'Export' and save the XAR package
5. Create an empty subwiki
6. On the subwiki, go to Administer Wiki > Content > Import
7. Import the previously exported XAR package

EXPECTED RESULTS

The XAR package is imported properly into the new subwiki.

ACTUAL RESULTS

There are times when the exported .xar cannot be imported in a new subwiki due to the following error: 

 2018-11-02 15:53:03.387:WARN:oejs.HttpChannel:qtp662441761-16: /xwiki/wiki/subwikiadmin/import/XWiki/XWikiPreferences org.eclipse.jetty.http.BadMessageException: 400: Unable to parse form content 

So far, I could not reproduce this error consistently. Last time I reproduced it on Oracle DB.