vB 5 Upgrade error

[iA1]

I am performing a test upgrade from vB 4.2.5 to vB 5.5.3 in order to make sure everything works fine and prepare for later use. The process aborted with a fatal error at 5.2.0 Alpha 3. It says:

520a3 Step #2
Fatal Error Occurred

internal error on /var/www/vhosts/testdomain.tld/httpdocs/core/vb/xml/import/theme.php : 537

Script: 520a3
Step: 2

Does anyone know how to resolve it and move forward?

Line 537 of theme.php says:

PHP Code:

// if we can't create the default parent, we don't want to recurse forever.
throw new vB_Exception_AdminStopMessage(array('theme_failed_to_create_parent_readonly', $insertResult['error'])); 


I guess the error is related to folder permissions but I could be wrong. All folders are 755.