Unpriviliged container wont start - Failed to allocate manager object

I wonder if you actually logged-in as lxcuser or if you did sudo su lxcuser.
Because I see a # and then a $ as prompt in your posted output, that could be explained by a switching from root to a standard user in this way. This could explain the lack of user idmap in your trace.
In this case it could be interesting to try again the arch container but with logging in directly as the user.