Too many redirects after logging in

I have a strange issue. Website is https://www.dgzn.nl

This is a membership site that I’ve ‘locked’ with Ultimate Member. Anyone can open this login page, but only Mac and iPhone users have problems logging in.
They either see a blank page or a ‘Too many redirects’ error message.
But when they refresh the page, they see their own profile, meaning they are logged in.

I am using htpps and post title as permalink.

Log in per PM if needed.
Thx for your time,
Rob