Drupal 7.69, Civi 5.23.0
A colleague just had an issue with CiviMail. She was trying to send a mail to Group "Volunteers" but exclude people in Group "Garden Volunteers". Instead of saying the number of recipients, it just froze on "Estimating..."
It still says estimating even if saving the draft and exiting.
When we try to submit the mailing, it gives a DB Syntax error.
I checked the Drupal error log and saw two relevant errors.
I ended up fixing this by just creating a new mailing with the same settings and it worked fine, but would love to work out why the initial issue happened so I Can fix it pre-emptively in case it happens again in the future.
Flushing cache did not solve it.
EDIT It's happened again, looks like it's just an issue with one particular smart group. We've definitely mailed to this smart group in the past though.
Someone previously asked what I see when clicking on the spanner icon: