Check out this support thread: https://toolset.com/forums/topic/call-to-undefined-function-wp_installer/
The reported error is almost exactly the same as mine, except for #6, the error is in woocommerce instead of Jetpack. So, the issue appears to happen with more than just Jetpack. Maybe this will help further troubleshoot?
There are now some errors in the log, but not the ones we are looking for:
PHP Warning: mysqli_real_connect(): (HY000/2002): No such file or directory in .../wp-includes/wp-db.php on line 1548
Thank you also for that other thread, I will keep an eye on it.
It is still not clear how this error happens.
I will keep the error log under observation as well.
You found that error on the test site?
I received a very similar error yesterday on my site:
PHP Warning: mysqli_real_connect(): (HY000/2002): Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (2) in /home/SITE/public_html/wp-includes/wp-db.php on line 1548
It occurred several times within an hour. None today; just the usual error (16 times so far). Could there be some connection? Strange that you saw these errors after I connected to Jetpack and imported my Toolset settings.
Here's one more thread reporting the same error, but they specify that ajax forms are generating the error: https://toolset.com/forums/topic/after-last-update-all-my-ajax-forms-gives-error-in-the-first-run/. Are there any ajax forms on the Test Site? I can't think that I have anything using Ajax on my example site (especially since it is behind a Coming Soon page, but maybe there is something behind the scenes). For what it is worth, I see the error much more frequently on my live sites than on the Coming Soon site.
I went back to the Test Site and added a Jetpack Subscribe and WP Views widget to the sidebar in case these make a difference.
This issue is finally isolated and resolved.
The fix will be available in the upcoming release (due next week, approximately)
Please let me know if I can help you in anything else!
This error was due to Jetpack performing background synchronizations.
We add a fix for this error in the Types version that has been released yesterday.
Can you update to 2.2.17 and re-check?
The particular error should not exist anymore.
Note that only this particular errors has been addressed.
You may still find errors in CRED screens related to Toolset.
If so, please open a new report for any other kind of error you may find.
Thank you!
Yes, it appears to be resolved! I updated both Types and Views on my sites (updating just Types but not Views 2.5 ended up with new errors!) and kept an eye on the error logs over the course of several hours. The error has not appeared since.
Thank you!