This is an error generated by PHP itself that can't move the file from it's temporary location into it's final destination.
You should check if you have enough disk space and if the destination folder is really writable (0777) by the web server.
@Stanislav - /tmp is the temporary folder where usually temp files go into, but depending on your php settings, it can be any other folder too.
Go to mailwizz > backend > misc > php info and look for the path where uploads go and make sure that is writable by the web server.