Hi Jan,
We are not really sure whether lightMart will get an update or not for J4.x. As you might aware, LghtMart is a pretty old template, we did update it to make it competible with the latest release, but in the base, it is an old template. Forking an older template to make it compatible with the latest release is just postponing the problems. LightMart can't even catch the flexibility of Joomla 3.9.x, no need to mention, Joomla4.0 is going to be a huge update and it might be impossible to make LightMart compatible with it even if we want to.
We are currently working on a new Joomla Template that will rock the market and will be the lightest & fastest and most enriched Virtuemart+Joomla Template that no body has ever seen ;) So, to be honest with you, all of our focus is currently on the new template we are developing.
To fix the registration problem that you encounter on VM3.4.2, simply follow these steps:
1- open this PHP file:
/root/templates/flexible_lightmart/html/com_virtuemart/user/edit_address.php
2- find these lines:
JHtml::_ ('behavior.formvalidation');
JHtml::stylesheet ('vmpanels.css', JURI::root () . 'components/com_virtuemart/assets/css/');
3- replace them with these:
vmJsApi::vmValidator();
vmJsApi::css('vmpanels');
4- then within the same file, find this line and remove it:
that's it. The registration page should start working just fine.
Regards.