Recently I moved an installation of Joomla! with VirtueMart to a new web host. The previous web host had restricted most of the export functionality in cPanel so I had to FTP everything over to the new host manually.
This created a lot of extra headaches. The most persistent was not being able to consistently save/update/create users in the administrative back-end. Every so often the form would time out and give the error message ‘Error Loading ModulesMySQL server has gone away’.
It took around 24 hours of constant searching, but I finally found a solution that worked for me at the Joomla forums.
Basically it states that the configuration.php needs to be edited so that it is in UTF-8 encoding without BOM. You can open the file in Notepad++ and set the encoding to this, then upload it and VIOLA.
Hopefully this saves someone else a headache.