Hi,
It's peculiar we have not thought about that earlier.
Yes... We were focused on an error and we didn't though that you might disabled that plugin.
Nicolas made a patch few weeks ago in order to check that the plugin is right enabled ; so HikaShop 2.5.1 will "fix" that kind of issue.
About the role of that plugin:
HikaShop User plugin to automatically synchronize the Joomla Users and the HikaShop ones
[/code]If a Joomla user is created in your website, the plugin will create en entry in his user table in order to store the data related to HikaShop.
Can you provide more details about the failed synchronization ? (so we can see to reproduce it...)
Regards,