Hello Mohamed - thanks for your efforts. Unfortunately this does not seem to be the solution, apart from the fact that I could not implement it.

In the meantime I have found that the problem seems to go deeper. Because when I look at the contacts in Odoo 11 - regardless of the website problem - they are sorted exactly as I described above for the listing of members on the website in Odoo 11. 

The same applies to the sorting of the contacts in the contact view in Odoo 13 - i.e. there the contacts are sorted differently than in Odoo 11 - exactly as I described it for the listing of the members in Odoo 13 on the website. In my opinion this sorting is wrong because it should be alphabetically independent of upper or lower case letters and the German letters "ÜÖÄüöä" should not be sorted to the end of a list ...

By the way - I have not adjusted / changed any sorting settings for the contact masks in Odoo 11 or Odoo 13 - everything is standard.

Hello Christian,

The membership records, by default, are ordered by id. Please refer to addons/membership/models/membership.py --> membership.line model:

class MembershipLine(models.Model):
    _name = 'membership.membership_line'
    _rec_name = 'partner'
    _order = 'id desc'
    _description = 'Membership Line'

To achieve your need, you can override the members controller on the module website_membership and add the order attribute to search method call when searching membership lines. (website_membership/controllers/main.py --> members method)

I hope that my response will be helpful for you.

Good luck.