500 Error on adding a member in a multisite configuration


When I try to add a member to a Standard Membership plan, I get Internal server error (code 500).
Here are the steps to reproduce the problem.
I created the following following configuration:
1. installed wordpress multisite with 1 site: siteA
2. created userA on siteA
3. installed and activated Membership 2 Pro plugin on siteA (I activated it individually, so not network activated)
4. created a membership on siteA and added successfully userA to it
5. created siteB
6. activated Membership 2 Pro plugin on siteB
7. created membershipB (Default type) plan on siteB
8. added userA as member of siteB
9. added userA as a member to membershipB, which resulted in Error 500

I would like to mention that adding newly created user to membershipB works, the problem occurs only for existing users.

Could you help me fixing this problem?

Thank you,