Today I've been trying to get all of my known CiviCRM on Joomla probelms documented as issues to help improve the CiviCRM Joomla experience. One glitch that I'm trying to investigate is that on my production site since installing Mosaico (and Flexmailer) I've had a problem with each CiviCRM version upgrade. The CiviCRM Database Upgrade Utility fails unless I disable Flexmailer (and Mosaico) first.
Most recently, going from 5.16.1 to 5.16.3, after installing 5.16.3 I didn't click on the CiviCRM Database Upgrade Utility link this time but instead went to Administer > Administration Console > System Status, then clicked on the alert for "Database update required". That gave me the following error:
Class 'Civi\FlexMailer\Services' not found
After disabling Mosaico and Flexmailer I was able to proceed with the database upgrade and it completed successfully.
On other occasions I have usually clicked the CiviCRM Database Upgrade Utility link. I don't know if that was the difference, but previously the database upgrade started but then stalled part way through. I didn't manage to reproduce it this time and couldn't find anything in the log.
This comment is I think regarding the same problem.
Because I can't reproduce this on a clean installation I haven't raised an issue yet. Can anybody else confirm if this is a problem for them currently?
I have Flexmailer 1.1.0 and Mosaico 2.1.1559427853 installed and Joomla 3.9.11.