Error when upgrading from 3.0.3.2 to 3.0.3.6

Hello:
I hope anybody can help!!
I have upgraded my cart from version 3.0.3.2 to version 3.0.3.6 using the softaculous installer in cpanel.
The php version was upgraded from v 7.2 to 7.3 prior to the upgrade.
the upgrade gave me an error:-

SyntaxError: Unexpected token < in JSON at position 0 OK Notice: Undefined property: Proxy::upgrade in /home/frequencyhealing/public_html/system/engine/action.php on line 79

Any help will be much appreciate as the cart is now showing an 404 error

Have you cleared all cache? Let us know if it's still happening after that.