Paypal redirect link after buying s2member subscription not working

[whistlersguild]

Hi,

We are building a subscription membership feature on our website and we are using the s2member plugin for wordpress. I've linked everything I believe I need to in order for the website to communicate to paypal properly. I can click the paypal button and pay for the subscription successfully, but after payment, the redirect back to my website doesn't work. I believe this redirect needs to work in order for the registration email to be sent through s2member. Without that, subscribers can't access the members-only content on the website. 

We're trying to launch this membership soon, so this is urgent. 

website:

https://whistlersguild.org/memtesting

s2member paypal button: 

[s2Member-PayPal-Button level=“1” ccaps="" desc=“Guild Member / $25 USD per year for full member access ($15 for first year)” ps=“paypal” lc="" cc=“USD” dg=“0” ns=“1” custom=“https://whistlersguild.org” ta=“15.00” tp=“1” tt=“Y” ra=“25.00” rp=“1” rt=“Y” rr=“1” rrt="" rra=“1” image=“default” output=“button” /]

redirect link snippet (with ...'s to reduce size): 

https://whistlersguild.org/?s2member_paypal_return=1&s2member_paypal_return_tra=ZGVmN...&token=1F.......

When it first tried the redirect link, the webpage broke and said:

Not Acceptable! An appropriate representation of the requested resource could not be found on this server. This error was generated by Mod_Security.

I worked with my hosting provider's support to try and fix the issue. I think they got me passed that part, but now after loading for a long time, the link above fails with this message:

[ whistlersguild.org ] says… ERROR: Unable to verify $_POST vars. Please contact Support for assistance. This is most likely related to an invalid configuration of s2Member, or a problem with server compatibility. If you are the site owner, and you're absolutely SURE that your configuration is valid, you may want to run some tests on your server, just to be sure $_POST variables are populated, and that your server is able to connect/communicate with your Payment Gateway over an HTTPS connection. s2Member uses the WP_Http class for remote connections; which will try to use cURL first, and then fall back on the FOPEN method when cURL is not available. On a Windows server, you may have to disable your cURL extension; and instead, set allow_url_fopen = yes in your php.ini file. The cURL extension (usually) does NOT support SSL connections on a Windows server. Please see this thread for details regarding the ideal server configuration for s2Member.

This is the link they point to: https://www.s2member.com/forums/topic/ideal-server-configuration-for-s2member/

I don't think the s2member support will help me fast enough, so really hoping the paypal community knows what I can do to resolve this. 

Any suggestions are much appreciated!