Support

This is a bug in AcyMailing. We already reported it to them on October 20th. They have already fixed it and told us they will include this fix in their next release. In the meantime, they told us that you should disable the system plugin "AcyMailing - Override Joomla emails" as a temporary solution.

I am only conveying what AcyMailing's support told us.

If you do not have that plugin enabled, please contact AcyMailing with a link to this ticket so they can take a look at the debug trace.

It looks like they have other Joomla 5 compatibility issues in their plugins beyond the one we had already reported to them.

It looks like they are not Joomla! 5 compatible yet. From a cursory look at the backtrace, they are using the obsolete JPlugin class. This class was deprecated since Joomla! 3.8.0 which was released on September 18th, 2017. They should've moved to the new \Joomla\CMS\Plugin\CMSPlugin class sometime in the last 6 years. The fact they haven't tells me they have a lot of legacy code in their extension which was only working on Joomla! 4 thanks to the modicum of backwards compatibility with Joomla! 3. This was removed in Joomla! 5 and things started breaking.

I can't speak for other developer, so I will tell you how we do things around here. We try to follow the deprecations and changes introduced into Joomla! every so often and put them on our roadmap. We are continuously paying the technical debt. When a new major version comes most of the technical debt is already paid, therefore we can focus on the changes introduced without much warning, bugs and regressions in the new Joomla! major version etc. Instead of struggling to catch up, we just try to figure out everything that was impossible to catch in advance. This makes for a far more enjoyable experience for us, and our clients.

You have found a major issue in Joomla!.

Joomla's “Behaviour - Backward Compatibility” plugin is not loaded by Joomla's CLI application before it tries to load system plugins. As a result, any system plugin which needs the backwards compatibility plugin to operate will break and bring down the Joomla! CLI application altogether.

I was able to trigger a minimal reproduction with a dummy plugin that's a single line of PHP code and I've traced the core Joomla! code. It's a trivial thing to fix and they would have caught it had they actually tested their backwards compatibility plugin under all Joomla! applications (frontend, backend, api, and cli).

The only thing I can tell you is to wait until Joomla! finally fixes this bug.

Yes, it's best to wait. Unfortunately, a lot of third party extensions still need the backwards compatibility plugin, and I understand why. For what it's worth, I plan on updating this site to Joomla! 5 in January or February. Everything is ready, I just want to make sure that little quality of life bugs and unexpected major issues, like this one, are addressed. I have already updated my other, less demanding sites such as my blog. So, I'd recommend doing all your due diligence to make sure that you are ready to upgrade the other sites and wait a couple of months for these early adopter issues to be ironed out.