Here’s a guide to debug if you have bug on your WordPress website:
- Make sure you have full backup of your site.
- Deactivate all your plugins.
- I mean ALL your plugins.
- Yes that contact forms and Jetpack too.
- Yes. ALL PLUGINS.
If the bug is gone after you deactivate all your plugins, you have bug in one of your plugins.
Activate your plugins one by one to get the buggy plugin. After you find it, contact the plugin developer.
If the problem is still there after you deactivate all plugins, switch to one of the default theme ( twentyfifteen, twentysixteen, etc ).
If after you switch to default theme the problem is gone, the bug is in your theme. Contact the theme author.
If the bug is still there after you switch to default theme. The problem is with your WP install/server.
Try to reinstall WordPress from Dashboard > Updates screen, if fails contact your host.
(updated on 03.DEC.2016)