Problem connecting to payment processor
This seems like it might be your CiviCRM not having a single canonical URL.

CiviCRM form processing requires a canonical URL to be set. If your site is configured to use http://example.org and the visitor lands on http://www.example.org, CiviCRM's form processing will redirect them to the configured base URL part way through the contribution flow and fail (because the user's cookies no longer are supplied).

I can't load the URL you've linked to, but if I start the Join process at the first URL below, I am able to get to PayPal. If I start at the second URL below, I see the "too many redirects" notice you describe.


http://isebiogeochemistry.com/index.php/iseb-membership
http://www.isebiogeochemistry.com/index.php/iseb-membership


Finding your site via Google ("ise biogeo chemistry") leads me to the latter hostname.

To prevent this (mis) behaviour, both the protocol (http vs https) and the hostname (example.org vs www.example.org) must be consistent (ideally, enforced with a redirect from ANY wrong URL to the single "correct" one).

How to configure canonical URLs varies between webserver and CMS, so I won't document that here. You'll find plenty of good existing docs with a search for eg "(your CMS) + canonical URL plugin", or "(your webserver) canonical URL".

Some similar questions on CiviCRM Stack Exchange:


**Network Error - Unable to reach the server
Network Error due to Cross-Origin Request Blocked
Payment options do not display when https used only when http used
&quot;Could not find a valid session key.&quot; Unable to save any admin forms