Login failiure for members, sometimes...

Hi,
I've noticed that the member(s) gets a Login failiure when they try to login to the site, and/but sometimes it works fine...

I have now set the login page "never to cash" in wp rocket, but I don't know if that will do any change? Can I do something else? It's important to my client that this works flawless.

The client have set 1 membership for each course they have. Could that be the reason?
Can several members be loged in at the same time, with the same membership account? The member can only see a specific page with pdf's to download.