When i upgraded to 3.5.1 I had to disable w3 total cache or I kept getting 500 errors. I also have on 1 server a multisite with domain mapping and single site. The domains mapped to the multisite responded with the single site install.
phpRedisAdmin is also getting 500 errors
Can you put your error log here for your sites.
Also can you try following if this solves your issue.
apt-get install php5.6-mbstring php5.6-bcmath
Hi all!
I am getting 500 server errors also on several customer sites. content isnt displayed completely, login to wp backend not possible for vhost sites. Seems database connectivity is kind of corruptd or so. the data is there though. I checked the databases.
All errors are related to ngx_pagespeed module:
2016/02/25 08:27:38 [error] 20098#20127: [ngx_pagespeed 1.9.32.6-7321] AprMemCache::Put error: Could not find specified socket in poll list. (70015) on key v2/mydomain.com/http://sub.mydomain.com/, value-size 174
2016/02/25 08:30:20 [error] 20095#20165: [ngx_pagespeed 1.9.32.6-7321] AprMemCache::Get error: Connection refused (111) on key rname/aj_RlizCtitY4n8Xj61kmss/https://www.mydomain.com/@@_
2016/02/25 08:30:20 [error] 20095#20165: [ngx_pagespeed 1.9.32.6-7321] AprMemCache::MultiGet error: Internal error (20014) on 1 keys
2016/02/25 08:30:20 [error] 20095#20165: [ngx_pagespeed 1.9.32.6-7321] AprMemCache::MultiGet error: Internal error (20014) on 1 keys
2016/02/25 08:30:20 [error] 20095#20165: [ngx_pagespeed 1.9.32.6-7321] AprMemCache::MultiGet error: Internal error (20014) on 1 keys
2016/02/25 08:31:54 [error] 20094#20094: [ngx_pagespeed 1.9.32.6-7321] AprMemCache::Get error: Connection refused (111) on key prop_page/https://wowstyles.de/_Jk7IqCNFGH@Desktop@beacon_cohort
2016/02/25 08:32:38 [error] 20094#20186: [ngx_pagespeed 1.9.32.6-7321] AprMemCache::Put error: Could not find specified socket in poll list. (70015) on key v2/mydomain.com/http://sub.mydomain.com/, value-size 174
2016/02/25 08:35:22 [error] 20094#20186: [ngx_pagespeed 1.9.32.6-7321] AprMemCache::Put error: Could not find specified socket in poll list. (70015) on key v2/mydomain.com/https://www.mydomain.com/wp-cron.php?doing_wp_cron=1456385721.8544330596923828125000, value-size 174
and so forth…
We have not updated anything in our NGINX modules. Also your issue looks similar to
please try restart nginx and memcache servers
service nginx restart && service memcached restart
Oh yes I am sorry, that is my main website (or actually first site created on that server. It has apart from the memcached and ngx_pagespeed isse no further issues.
This is also true for my other servers: the first created site has no issues. However the other sites that followed do have issues. And this is for example the error.log of one of the vhost sites:
error log for vhosts, that are getting 500 on wp-admin and where frontend content isnt visible in total:
PHP message: PHP Stack trace:
PHP message: PHP 1. {main}() /var/www/domain.de/htdocs/index.php:0
PHP message: PHP 2. require() /var/www/domain.de/htdocs/index.php:17
PHP message: PHP 3. require_once() /var/www/domain.de/htdocs/wp-blog-header.php:16
PHP message: PHP 4. include() /var/www/domain.de/htdocs/wp-includes/template-loader.php:89
PHP message: PHP 5. get_header() /var/www/domain.de/htdocs/wp-content/themes/twentyfifteen-child/single-post.php:10
PHP message: PHP 6. locate_template() /var/www/domain.de/htdocs/wp-includes/general-template.php:45
PHP message: PHP 7. load_template() /var/www/domain.de/htdocs/wp-includes/template.php:521
PHP message: PHP 8. require_once() /var/www/domain.de/htdocs/wp-includes/template.php:562
PHP message: PHP 9. get_sidebar() /var/www/domain.de/htdocs/wp-content/themes/twentyfifteen-child/header.php:46
PHP message: PHP 10. locate_template() /var/www/domain.de/htdocs/wp-includes/general-template.php:125
PHP message: PHP 11. load_template() /var/www/domain.de/htdocs/wp-includes/template.php:521
PHP message: PHP 12. require_once() /var/www/domain.de/htdocs/wp-includes/template.php:562
PHP message: PHP 13. dynamic_sidebar() /var/www/domain.de/htdocs/wp-content/themes/twentyfifteen-child/sidebar.php:28
PHP message: PHP 14. call_user_func_array:{/var/www/domain.de/htdocs/wp-includes/widgets.php:729}() /var/www/luciaschwalenberg.de/htdocs/wp-includes/widgets.php:729
PHP message: PHP 15. WP_Widget->display_callback() /var/www/domain.de/htdocs/wp-includes/widgets.php:729
PHP message: PHP 16. Jetpack_Subscriptions_Widget->widget() /var/www/domain.de/htdocs/wp-includes/
2016/02/25 08:42:50 [error] 20098#20098: *121 FastCGI sent in stderr: “PHP message: PHP Fatal error: Call to undefined function utf8_encode() in /var/www/domain.de/htdocs/wp-content/languages/de_DE.php on line 62
PHP message: PHP Stack trace:
PHP message: PHP 1. {main}() /var/www/domain.de/htdocs/wp-cron.php:0
PHP message: PHP 2. require_once() /var/www/domain.de/htdocs/wp-cron.php:26
PHP message: PHP 3. require_once() /var/www/domain.de/htdocs/wp-load.php:42
PHP message: PHP 4. require_once() /var/www/domain.de/wp-config.php:56
PHP message: PHP 5. require() /var/www/domain.de/htdocs/wp-settings.php:350” while
reading response header from upstream, client: 37.120.189.189, server: domain.de, request: “POST /wp-cron.php?doing_wp_cron=1456386170.4892580509185791015625 HTTP/1.0”, upstream: “fastcgi://127.0.0.1:9000”, host: “domain.de”
2016/02/25 08:42:51 [error] 20098#20098: *119 FastCGI sent in stderr: "PHP message: PHP Fatal error: Call to undefined function xml_parser_create() in /var/www/domain.de/htdocs/wp-includes/class-IXR.php on line 264
PHP message: PHP Stack trace:
PHP message: PHP 1. {main}() /var/www/domain.de/htdocs/index.php:0
PHP message: PHP 2. require() /var/www/domain.de/htdocs/index.php:17
PHP message: PHP 3. require_once() /var/www/domain.de/htdocs/wp-blog-header.php:16
PHP message: PHP 4. include() /var/www/domain.de/htdocs/wp-includes/template-loader.php:89
PHP message: PHP 5. get_header() /var/www/domain.de/htdocs/wp-content/themes/twentyfifteen-child/image.php:10
PHP message: PHP 6. locate_template() /var/www/domain.de/htdocs/wp-includes/general-template.php:45
PHP message: PHP 7. load_template() /var/www/domain.de/htdocs/wp-includes/template.php:521
PHP message: PHP 8. require_once() /var/www/domain.de/htdocs/wp-includes/template.php:562
PHP message: PHP 9. get_sidebar() /var/www/domain.de/htdocs/wp-content/themes/twentyfifteen-child/header.php:46
PHP message: PHP 10. locate_template() /var/www/domain.de/htdocs/wp-includes/general-template.php:125
PHP message: PHP 11. load_template() /var/www/domain.de/htdocs/wp-includes/template.php:521
PHP message: PHP 12. require_once() /var/www/domain.de/htdocs/wp-includes/template.php:562
PHP message: PHP 13. dynamic_sidebar() /var/www/domain.de/htdocs/wp-content/themes/twentyfifteen-child/sidebar.php:28
PHP message: PHP 14. call_user_func_array:{/var/www/domain.de/htdocs/wp-includes/widgets.php:729}() /var/www/domain.de/htdocs/wp-includes/widgets.php:729
PHP message: PHP 15. WP_Widget->display_callback() /var/www/domain.de/htdocs/wp-includes/widgets.php:729
PHP message: PHP 16. Jetpack_Subscriptions_Widget->widget() /var/www/domain.de/htdocs/wp-includes/class
And so on…
Seems as if the php compilation has no xml support. And I must correct my initial statement, that my main websites dont have these issues. They all do. I checked their individual error.logs as well. First posted log was nginx general error log.
I already did that and was about to post this solution here as well! 
Thanks for replying so quickly this morning!