Strange. The most common reason behind a white screen like that would be a php fatal error. Can you try this please:
- Go to the forum admin > toolbox > Error Log and clear any entries using the button at the bottom.
- Attempt to perform the upgrade.
- Look at any source code that may have started to display. Depending on the browser you use this might be called ‘View Source’ on one of the browser menus.
- If anything is listed it may be a php ‘Fatal Error’. If so – can you copy the message of the error text and post it here.
- If nothing in the source – go back to the forum error log to see if any fatal error (shown in pink) has been recorded.
If no actual errors in the error log then you may have to take a look at the php error log in your server. again looking for a ‘fatal error’ entry. Normally there would be a way to view the most recent entries via your hosting control panel.