524 Error for one customer

Hi guys,
I have a strange issue with cloudflare on one of my websites.

Namely we received an email from a customer. She’s not able to log in to her account.
Every time she tries to do that she gets the 524 error on the website.
It is very strange, because the problem affects only her account.
I purged cache on cloudflare but it didn’t help :confused:
We deleted her account and created a new one with the same email address and the error repeats.
When we were logged to the new account everything worked fine. After she logged in to it the error returned.

Would you be able to help me with this problem?
Cheers
J