Can't create or update members Membership2

Actually the Membership2-Plugin doesn’t work on my site.
If it comes to the point assignment of a membership, after the second click on save_changes I always get Unknown error type: [2] Creating default object from empty value. I have already inform the plugin creator via mail.

Now I figured out what is the source.
The return of get_subscription is null.
So it seems that the subscriptions aren’t loaded properly, because the issue occure on members with an active membership as well.

https://www.ads-software.com/plugins/membership/