Membership 2 Pro login doesn't work

I used the shortcode [ms-membership-login] on the page member-login on https://securi-finder.nl/member-login. Everything is usualy working just fine, but it seems like the page is reloading itself and then comes back with a failure message.

I can't login here with any account, still I am able to login via the usual way https://securi-finder.nl/login. I need to membership login to work again.

I had this problem before, I was going to report the issue here, but it solved itself.
After that it worked like another week, and now the issue is happening again.
Can you take a look at what might be the problem?

I opened the securi-finder.nl support door, so you can enter whenever needed..
Thanks in advance,
Henk Rensenbrink.