The more active plugins and more active features, then more needed memory allocation. The update may contain new features that trigger the memory issue. The solution would be turning off other plugins to give more room to new main features or increase the memory limit.
It's the same when new versions of software or game, it may require higher specs in the next coming versions depending on new features
I'm not saying it's the issue on your second site is the same, but please try that as well. Check the fixed first site's
wp-config.php through FTP, and check how memory limit and WP_DEBUG is set, then try applying the same setup in your second site's wp-
config.php. Please don't copy the entire content, just the line WP_DEBUG and MEMORY LIMIT.