502 Gateway Error – Site unusable

This theme continues to generate a 502 gateway error when anyone logins using the /wp-login.php page.

Admin log is possible using /wp-admin

It has effectively made the site unusable since I can’t complete the site and launch it while this error persists.

Obviously I’ve raised a ticket but the past two responses (taking two days) have been unhelpful and now it’s been ignored for another 24 hours. So no progress for three days now.

The hosting company have proved it’s not plugins by disabling them all, but when they disabled the dating theme and used WordPress 2018 theme instead the error disappeared, so there’s a clear indication that the fault is with the theme.

The site is running on PHP v7.3 and it’s a clean, fresh install.

Anyone got any ideas how to fix this?