I’m going to guess that PHP’s settings have changed with the upgrade. Division by zero suggests some variable is not initialized (and is being dynamically typed as 0). The PHP manual has a lot of information on the changes between 5.x and 7.x. This bit on how variables are handled supports my hypothesis, especially if your theme was created before 7.x was released.
On the same lines, I am going to guess that PHP is running out of memory (because the new settings file sets the value too low by default for your theme). I am guessing this is the case because the lines from the error log do not appear to be FATAL but rather, WARNING (based on your comment). When PHP runs out of memory, it sometimes doesn’t even have enough to write to the error log, which makes debugging this type of error very difficult.
I suggest you do three things:
- Contact the theme author and see if they have an updated version of the theme available (assuming it is not your own).
- Increase the amount of memory PHP can use to something like 64MB or even 128MB and see if you can reproduce the error. You can do this in a variety of ways. See this link for more info.
- Turn display_errors off and instead send them to a log file so your users don’t see PHP errors when using the site. Put the code snippet below in wp-config.php just before the STOP EDITING HERE line:
define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true );
define( 'WP_DEBUG_DISPLAY', false );