PHP fatal error on server after deactivating modules Today our site went down with only white pages showing. The logs from the nginx server looks like this: 

[01-Sep-2016 10:33:15] WARNING: [pool www0] child 26970 said into stderr: "NOTICE: PHP message: PHP Fatal error:  Class 'MigrateSimpleFieldHandler' not found in /srv/www/mysite/public_html/sites/all/modules/entityreference/entityreference.migrate.inc on line 18"
[01-Sep-2016 10:33:15] WARNING: [pool www0] child 26970 said into stderr: "NOTICE: PHP message: PHP Fatal error:  Class 'MigrateUIWizard' not found in /srv/www/mysite/public_html/sites/all/modules/migrate_d2d/migrate_d2d_ui/migrate_d2d_ui.migrate.inc on line 18"
[01-Sep-2016 10:33:20] WARNING: [pool www1] child 26712 said into stderr: "NOTICE: PHP message: PHP Fatal error:  Class name must be a valid object or a string in /srv/www/mysite/public_html/includes/common.inc on line 8026"
[01-Sep-2016 10:33:24] WARNING: [pool www1] child 26710 said into stderr: "NOTICE: PHP message: PHP Fatal error:  Class name must be a valid object or a string in /srv/www/mysite/public_html/includes/common.inc on line 8026"
[01-Sep-2016 10:34:27] WARNING: [pool www1] child 27190 said into stderr: "NOTICE: PHP message: PHP Fatal error:  Class name must be a valid object or a string in /srv/www/mysite/public_html/includes/common.inc on line 8026"
[01-Sep-2016 10:34:40] WARNING: [pool www1] child 26712 said into stderr: "NOTICE: PHP message: PHP Fatal error:  Class name must be a valid object or a string in /srv/www/mysite/public_html/includes/common.inc on line 8026"
[01-Sep-2016 10:34:56] WARNING: [pool www0] child 23319 said into stderr: "NOTICE: PHP message: PHP Fatal error:  Class name must be a valid object or a string in /srv/www/mysite/public_html/includes/common.inc on line 8026"


This happens as soon as I try to update the database, I also get a "502 Bad Gateway" message from the server just before the screen turns white.

Should I activate the deactivated modules again or uninstall them?

EDITED: I can also mention that according to the status report there are database updates waiting to be run, but when I do that locally (wamp) I get the message that no database update is required. When I try to do this db update on the web server (nginx) I get 502 bad gateway error and then the pages go blank.
Finally the problem is solved thanks to this thread.
The database update wasn't possible for the Field collection module (but it could have been any number of modules), but with the help of all the people that contributed to the solution in the thread above, I was able to get the db update to run as expected. After that everything seems to work.