Joomla Virtuemart registration and checkout problem

We have a website running on Joomla and the webshop powered by Virtuemart. Main problems: - Registration error: Users cannot register by Joomla default registration or VM registration page. VM registration leads into a 500 error page. Only username will be saved in system. - Checkout error: When checkout the basket, after final confirmation 500 error pages is shown. When refresh page, website with empty page is shown. Purchase order is made, but user doesn't see this, basket is still filled with products. During development of this site, we have made some hard cored changes in the php files. I don't have a summary of these changes. This problem is not caused by these changes, but occurred after a few months of normal use. I will not update the joomla and virtuemart versions at the moment. Virtuemart log file does not show anything weird. Disable Artio JoomSEF doesn't have effect. Disable Chaptha doens't have effect. Joomla 1.5 VM 1.1.8 stable