Hello,

Somehow the server's PHP open_basedir setting is misconfigured. If you look at the error message it only is including /tmp in the open_basedir.

I'm actually surprised it didn't fail earlier in the process, because PHP scripts are getting loaded up okay, but maybe that is due to some kind of caching allowing those to load regardless of the setting of open_basedir.

Regardless, I'm 100% certain this is a server misconfiguration and not a Composr bug. Check your open_basedir setting, or ask your webhost if it's not something you've configured.