That's
Bug 28221 - [Mac] shortcut "Alt+L" coflicts with "@" char on german mac keyboard fixed for GNR 6.0.
Workaround is to change the shortcuts for low priority:
/opt/zimbra/mailboxd/webapps/zimbra/WEB-INF/classes/keys ZmKeys_de.properties &
ZhKeys_de.properties
You could either create .mac values:
compose.LowPriority.display = Alt+L
compose.LowPriority.display.mac = Alt+value
compose.LowPriority.keycode = Alt+76
compose.LowPriority.keycode.mac = Alt+value
Or change it to what it's going to be for 6.0 and not mess with specific .os attributes:
compose.LowPriority.display = Ctrl+P,Ctrl+L
compose.LowPriority.keycode = Ctrl+80,Ctrl+76
compose.LowPriority.description = Low priority
compose.LowPriority.sort = 30110
su - zimbra
zmmailboxdctl restart