When you have problem with the registration in virtuemart type, you should check a native Virtuemart plugin, the VM Framework Loader during plugin updates settings, because it may cause the problem.

You need to navigate to extensions / plugins / system plugins and select the VM Framework Loader during plugin updates. Go to the secondt tab and set redirect parameter to no.

Import sliders
VM Framework Loader during plugin updates

After that you should check the login again.