Recently when installing MySQL workbench on a clean Ubuntu 20.04 install I've run into the following error when connecting locally using a stored password: "An AppArmor policy prevents this sender from sending this message to this recipient.."
This post on MySQL forum suggests its a permissions issue between MySQL Workbench and the password manager. The fix is quick and easy allowing the two to talk to each other.
sudo snap connect mysql-workbench-community:password-manager-service :password-manager-service
The snap store listing does call this out but if, like me, you installed it from the command line you might not notice:
If you use connections, Workbench use Password Manager and ssh to work properly. So it is necessary to give this permission explicitly. - snap connect mysql-workbench-community:password-manager-service - snap connect mysql-workbench-community:ssh-keys - snap connect mysql-workbench-community:cups-control
It's also possible to edit the permissions in the Snap Store UI by clicking Permissions: