1. Check for plugin conflicts:
Deactivate all possible plugins: Did that solve the problem?
If it did, it's a plugin conflict — reactivate plugins one at a time until the problem returns. The last plugin activated is the culprit.
2. Check for theme conflicts:
With all plugins disabled, switch to a bundled theme (2011, 2012, 2013, 2014). Did that solve the problem?
If it did, your theme is causing the issue.
3. Check for an issue with your hosting — try it on a different server or local install — if it works there, something is funky with the server setup.
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Nick Ciske
CTO / Director of Web Engineering
@nciske