It is likely a callback function for something. My guess is that somewhere something is trying to use a function named mytheme_admin_bar_render as a callback but that function has not been defined. Something like…
add_filter('something','mytheme_admin_bar_render');
… possibly but there are many, many possibilities.
grep your wp-content folder for mytheme_admin_bar_render, or use an IDE that can do source code searches over entire directories. It is hard to say what to do when you find it. If it is a filter, as above you can probably just comment it out or delete it.
// add_filter('something','mytheme_admin_bar_render');
If it is something more complicated, you may need a different approach.
Without the code for the plugin (and I can’t find it) solving the other issue isn’t going to be easy. Chance are that the plugin is trying to do something like …
foreach ($somearray as $k=>$v) {
//...
}
… where $somearray has not been set or is set to false or some other data type that you can’t foreach over. Look for the nearest foreach to that line in the error and try to analyze what is happening. Something like…
if (isset($somearray) && is_array($somearray)) {
foreach ($somearray as $k=>$v) {
//...
}
}
… would probably do it.