Upgraded to V7.12.6 multiple pages now blank

I’ve seen articles previously to do with a similar issue but they’re quite old. Not really relevant to current version.

Managed to upgrade to Version 7.12.6 which was exceptionally challenging, had to do it in stages and do some crappy hack in .htaccess file to get it to upgrade.

Since upgrading, I’ve noticed that there are several pages that won’t load.
All > Contacts, Accounts, Opportunities, Leads and many other pages.
It just shows a blank page with the footer.

I can create the above records but I cannot view or edit them once saved.

Any ideas please?

You should probably find some FATAL errors in your php_errors.log, please check and report back.