Caldcyn: Seems like there might be some other plugin conflicting. Do you think you could check what other plugins you have installed and then we can go from there to figure out with which one the conflict appears, and why.
Here are the plugins currently installed and active:
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Ad Inserter
AddToAny Share Buttons
Akismet
Content Progress
Easy Google Fonts
Enable Media Replace
Enhanced Text Widget
Get Custom Field Values
Google Analytics
Heartbeat Control
HTML Widget Text
Jetpack by WordPress.com
Limit Login Attempts
MailChimp for WordPress Lite
Media File Manager Advanced
Media File Renamer
NextGEN Gallery by Photocrati
PHP Code Widget
PHP Text Widget
Post Snippets
Public Post Preview
Social Media Feather
Subscribe to Comments Reloaded
Subscribe to Comments Reloaded Better Unsubscribe
Subtitles
TinyMCE Advanced
Weaver Xtreme Theme Support
There is a new version of Weaver Xtreme Theme Support
available. View version 2.0.1 details or update now.
WP Hide Post
WP-SpamShield
YAMB Show Posts
Yoast SEO
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I think the only one that uses the TinyMCE toolbar is the NextGen Gallery, and I tried disabling that, but it didn’t fix it.
When WordPress was upgraded to 4.4 (unfortunately automatically w/o my knowledge or ability to test first) I had other things break too, like php code in sidebar widget. I installed a different plugin to fix that. Like I said, I don’t know exactly when this broke because I don’t use it every day (probably haven’t used it in about 3 weeks). I wish WP had update log files!
Wish I knew more about behind the scenes to diagnose more! I know code, but I’m not too familiar w/ WP backend.
I finally had a chance to do some more investigating and found that the conflicting plugin is Media File Manager Advanced (v. 1.1.5).
I’m not sure what it is doing to cause the post-snippets-dialog not to load/initialize.
I could take a look at Media File Manager Advanced
and see what the problem is. Though that plugin seems to be removed from wordpress.org plugin repository. All links to it here are now dead it seems like.
Did you get it from somewhere else, or did it just recently get removed from here?