I've noticed the following lines showing up in my error log with PHP 7.0+:

PHP Warning:  A non-numeric value encountered in /.../wp-content/themes/atahualpa/css.php on line 137
PHP Warning:  A non-numeric value encountered in /.../wp-content/themes/atahualpa/css.php on line 138

I'm using Atahualpa version: 3.7.24.

I searched the forum -- apologies if this is duplicated in any other post. Thank you for all your help over the years.

Please go to ATO->Export/Import Settings - export your settings and attach them to a reply (use the paper clip icon)

Thank you juggledad. See attached Atahualpa settings.

Do you have any clue as to what was going on when the warning shows up?

where are you seeing that message i.e. what error log?

The error_log is generated in Wordpress Debugging Mode.

I think the error is generated when a viewer visits the site.

Do you have define( 'WP_DEBUG', false ); set in your wp_config.php?

I haven't been able to recreate this so I don't know what is causing it. If you can figure out what causes it, I can take a look and come up with a solution.

After a long while, I took another look at the error_log.

The errors disappeared when I added Layout MIN and MAX width numbers into the Style and Configure Layout Options.

When I removed the numbers and saved, the errors returned. So the issue appears to be that blank MIN/MAX Width layout options will produce these error messages for some reason.

Also, I did not mention this earlier but it seems like it could be relevant. There is one more error -- a PHP Warning -- that shows up just before the other errors I mentioned before.

[17-Dec-2019 10:50:20 UTC] PHP Warning:  call_user_func_array() expects parameter 1 to be a valid callback, no array or string given in /.../wp-includes/class-wp-hook.php on line 288
[17-Dec-2019 10:50:20 UTC] PHP Warning:  A non-numeric value encountered in /.../wp-content/themes/atahualpa/css.php on line 137
[17-Dec-2019 10:50:20 UTC] PHP Warning:  A non-numeric value encountered in /.../wp-content/themes/atahualpa/css.php on line 138

That PHP Warning error continued to show even after I took care of the issue causing the other errors.

[17-Dec-2019 11:15:17 UTC] PHP Warning:  call_user_func_array() expects parameter 1 to be a valid callback, no array or string given in /.../wp-includes/class-wp-hook.php on line 288
[17-Dec-2019 11:15:19 UTC] PHP Warning:  call_user_func_array() expects parameter 1 to be a valid callback, no array or string given in /.../wp-includes/class-wp-hook.php on line 288
[17-Dec-2019 11:15:27 UTC] PHP Warning:  call_user_func_array() expects parameter 1 to be a valid callback, no array or string given in /.../wp-includes/class-wp-hook.php on line 288

I'm running PHP 7.1.

Once again do you know what you are doing when that occurs?
What plugins do you have active?
What version of WordPress?

After much troubleshooting, I seem to have nailed down the problem to two plugins: "collapsing categories" (causing the first set of errors), and "collapsing pages" (causing the other error), both venerable old wordpress plugins.

Sorry for the disruption and thank you for your generous patience.

Glad you figured it out!