A very common issue with upgrades in some environments is the Ajax loading - error on upgrade (Core Joomla upgrade).
If you already tried the usual causes like php post_max_size and max_exec_time and you haven't figured it out why it still doesn't work try our next solution below.
Joomla 3.7 introduced the custom fields as part of the core. It was a great feature missing in previous releases. However you may experience some issues while upgrading your site and trying to use it.
If you face the "illegal mix of collations issue" and already tried to fix the collation in phpmyadmin or using AdminTools extension, you can try the solution below.