@optimizingmatters Hello and thank you for the response.
Here are some answers below. Just as an initial note – I am starting to think our specific site issue might be unrelated to your plugin. As I note below, I will update here if this is the case.
* _how_ does the CSS & JS break? do the files 404? or do they cause an internal server error?
Since the plugin’s functionality is completely disabled due to it being unable to write to the cache directory, nothing breaks or 404s. When you’re logged in to the site, you simply see an error stating:
Autoptimize cannot write to the cache directory (/srv/bindings/c6cf2ec7251745139b2efad85e1f6408/code/web/wp-content/cache/autoptimize/), please fix to enable CSS/ JS optimization!
* what happens if you turn CSS optimization on with the “inline all CSS”-option on?
Because the plugin’s functionality is disabled, you can not access any of the plugin’s configuration in the admin interface. So I can not view any options to toggle functionality and troubleshoot further.
* what happens if you disable “aggregate JS” and/ or “aggregate CSS”?
Same as above. The plugin is effectively disabled if it can not write to the cache directory.
* “after a recent update” of what? AO or your theme or a plugin?
I am now thinking that the error we’re seeing is either specific to this site’s setup or Pantheon environment. I realize this question was directed at the OP. In my case, I discovered the issue while updating WordPress to 5.2.2 and the Autoptimize plugin to 2.5.2.
If we discover that this error is due to a site-specific build setup (we use a composer-based continuous integration setup for this site) which may be causing this issue. We’re just trying to troubleshoot all options right now. I will post here if this was the cause of the issue on our end.
* any (relevant) errors in your PHP errorlog when this happens?
There are no relevant errors in our PHP logs.
* have you reached out to Pantheon and if so, what was their feedback?
I will do this next.
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Totally unrelated, but important – thank you for creating an awesome plugin and caring about front-end performance in WordPress.