Upgrade issues from 1.3.8a to 1.5.1

Hi all,
been using 1.3.8a for sometime now with no reported issues but thought it was time to upgrade for security reasons, wish I did not have to but hey ...

OK, so I followed instructions

1. Created new folder
2. made a full back up of my site including data base
3. Installed clean install of 1.5.1
4. Uploaded data base
5. Ran zc_install to update database
6. Tested and all working fine
7. Installed mods used in previous version 1.3.8a but used 1.5.1 versions and all good
8. Installed template used in previous version 1.3.8a and all worked fine but for some cosmetic issues -

except for this

LOG IN or CREATE ACCOUNT or CHECKOUT I get an error for this template ONLY

**SERVER ERROR
The website encountered an error while retrieving http://popperexpress.com.au/index3/index.php?main_page=login. It may be down for maintenance or configured incorrectly.
Error code: 500
**
I can add to cart no problems but cannot log in or create account or check out ? If I revert back to the classic green template I can perform all functions perfectly but as soon as i use the template from the previous version I get these issues.
I still have my site running on the previous version until I can get this ready to be swapped over, don't know if that is the issue but thought I would mention it never the less, oh and it is not down for maintenance either

http://popperexpress.com.au/ is my running site on 1.3.8a and http://popperexpress.com.au/index3 is the new version

What am I doing wrong ? Please help !!

If I revert back to the classic green template I can perform all functions perfectly
Well you apparently have one or more template override files that are not updated

You can try manually using FTP renaming them one at a time from xxxxxxx.php to xxxxxxx.off

Then checking functionality - rename back if no change and move on to the next one

Thank you for the reply Kobra but to be honest with you, that went straight over my head. I am not an expert by any means. Where do you find the template over ride files ? and can you give me an example of what you mean please.

I get a white screen when I go to http://popperexpress.com.au/index3/

sorry lat9 I was just trying to implement the suggestions of Kobra Please try it again as I have restored it

The pages you're having problems with are "supposed" to be https:// pages, but at least the login page link from the home page is an http:// link. Do you have SSL enabled in your /includes/configure.php file?

Hi lat9
Have tried with both SSL set to true and false but no luck. It is set to true for the moment in case that aids any one helping me solve this issue

Thank you to all those that tried to help me out but have decided to just employ a professional to do the job