We had a similar issue - lots of JS errors, “… is not a function” leading to display:none on form.
We were on version 2.5.8 and found that reverting to 2.5.6 did fix the issue.
However, we eventually discovered that we had previously added some custom code to move GF scripts around that we found here:
Removing the following hooks fixed the issue:
add_filter( 'gform_cdata_open', ...
and
add_filter( 'gform_cdata_close', ...
We kept the following line:
add_filter('gform_init_scripts_footer', '__return_true');
So it seems the advice in the above post is breaking something in recent releases of GF.
Hope that helps someone.