server error 500 when saving changes on 'Edit Member' page

Membership Pro Plugin is working perfectly on a live website except for one thing: When I go to ‘All members’ > ‘Subscription details’ I’m not able to save changes like ‘Expire Date’ made under ‘Manage subscriptions. Clicking the ‘save changes’ button will result in a 500 server error.

Url is like this: …/wp-admin/admin.php?page=membership2-add-member&user_id=367

Any idea how this is caused and how it can be solved?

I’m using version 1.0.2.1

Thanks in advance!