persistent 500-errors on registration - Membership

Heya,

So I've run into a problem with Membership.
I'm persistently getting 500 server errors whenever I try to register.

http://www.stichtingijsselwijs.nl/index.php/join/?action=registeruser&subscription=2(this is a test-subscription; /join/ is not linked to in any menu but the subscription plan is set to active and public)

Sometimes the error is thrown when clicking the 'Sign Up' button, sometimes itt akes you to the page where you input your name, pass and emailaddress.
Registrations ending in a 500-error this way are added to the backend with only a name, mailaddress and registration-IP.

I've already searched the internet and WPMUDEV in particular, mostly running into abandoned threads with kinda similar problems. The only real match would be this thread

Ofcourse, Ive tried all steps suggested in this (and linked) threads, including:
- firstly, I tried twentyfourteen with no other plugins active than Membership and Pronamic Ideal,
- check PHP memory (was set to 256M by default),
- change Form type from original (giving 500 errors) to popup (either displaying a blank page or a 'problem with registration' error popup),
- deleting the plugin files and all wp_m_* tables and re-installing,
- emptying the tables instead of dropping them,
- veryfying/repairing the tables via the plugin,
- switching around 'Use Subscription' from none to others,
- using the Duplicator plugin, I cloned the site to my local server where everything worked perfectly..

The last post in the linked thread poses me with 2 more questions:

"If you have problem with CURL, then it's must be hosting issue. You need to deal hardly with your hosting provider."
- Assuming my hostingprovider can fix this, exactly what do I have to ask them to change?

"Also, for free subscription did you enable free gateway? Would you please try with a paid subscription in paypal sandbox mode if that works (just to test)?"
- Which of these 2 paypal gateways is 'sandbox mode'? PayPal Express Gateway, PayPal Single Payments Gateway EDIT: I found it; did not help with the errors though

The error_log shows the following:
Premature end of script headers: php-cgi, referer: http://www.stichtingijsselwijs.nl/index.php/join/?action=registeruser&subscription=2
I am not sure it is related, but just to be complete, another error caught my eye as well:
ap_pass_brigade failed with error 103: Software caused connection abort

If you need any more specific info, don't hesitate to ask. I'll get back to you ASAP (probably on monday)

Thank you for your time.

Greets,
Kelvin