I am completely unable to access my site--front end and back end--after upgrading to v6. Do I have any option other than restore a backup using akeeba?
You could try to delete the Acymailing folders via ftp. Afterwards download the latest Acymailing package form Acyba's websitye and install it.
I think you have a plugin which is causing the issue.
If you didn't find any solution then feel free to contact us via the contact form.
We will take a look at it and fix everything 😉