Plugin is the another reason of 500 error . It could be happen for any specific plugin . For this deactivate all plugin .
=>> (Trick) If you can’t deactivate plugin normally user the trick – To deactivate plugin go to your c-panel file manager or from ftp go wp-content and change the name of plugin folder name.
After deactivate go to Dashboard and Activate plugin one by one.Then see for which plugin it is happening and delete the plugin (If you use Trick rename the plugin file before delete ) .
Sorry I forgot the rule , Jan Dembowski . Forgive me for my fault.