Advise turning on Debugging all the way to developer to see if moodle will report more info.
When you did the 'march' of your site, did you also have to upgrade PHP + extensions + add a PHP extension.
If you can, go to Site admin -> Server -> Environment and update the component. Then look at the results below to see if moodle has any REDS or YELLOWS.
3rd party plugins have been known to create havoc ... themes are plugins. If you are using a 3rd party theme for the site, suggest forcing the site to use boost - a theme that comes with core - by adding a line to config.php:
$CFG->theme='boost';
above the comments at the end of config.php file.
What other 3rd party plugins do you have installed? Are they up to date with current core version as well?
'SoS', Ken