Create Lead Form error

Hello SuiteCRM community,

The following error comes up when I try to create a lead form:

No data received

This is an example Create Lead form url that fails: index.php?module=Campaigns&action=WebToLeadCreation&return_module=Campaigns&return_action=index

I also get the same response when I try to select a security group, from,say, a campaign details page, e.g. index.php?module=Campaigns&offset=1&stamp=1455492844062100600&return_module=Campaigns&action=DetailView&record=deecbd3d-ff1e-7dd7-f9bd-56c10935433d

Version: 7.5.1

Have you considered it may be a security group issue?

Thanks for your response. I suspected that too. I am still using the Admin account. To prove your point, I have create an admin group, and an admin role with access to everything, then made the admin user a member, still no change. Let me know what info you need from my server so you can help with troubleshooting. I cannot bother trying out other features is the web to lead forms do not work. Thanks.

I have checked the site_url value, it is correct ad is using https.
My suitecrm is at a publicly accessible installation; running on a VPS.

I setup the permissions as follows:

Set the following permissions on the SuiteCRM directory(Linux):

sudo chown -R www-data:www-data .
sudo chmod -R 755 .
sudo chmod -R 775 cache custom modules themes data upload config_override.php

I see that another user had this issue as well, 10 months ago, but there is no solution there:

Please help me. If you feel the need to ask me questions, please ask me several questions so we can get to a quick resolution.