Hello Gergo - do you have a support / maintenance contract with Qlik - based on the question I would assume so unless you are using Qlik Sense server under a trial basis.
If you have support: Can you please open a support ticket to Support@qlik.com, it would be easier and confidential to shared logs and images, etc.
If you are using this as a trial - have you contacted the appropriate pre-sales contact assigned to assist you.
Please let us know. - Have your logs available.
Please mark the appropriate replies as helpful / correct so our team and other members know that your question(s) has been answered to your satisfaction.
Support hasn't come back to me yet, but the issue got sorted.
I just re-installed Qlik Sense on Server B, deleted all the certificates, all the files from C:\ProgramData and re-installed QlikSense. I could add Server B to the nodes without any problem.
I'm still curious what can cause this issues - this morning removed the node again just to test something else and couldn't add it back, needed to re-install Sense again...
Looking forward to support's answer.
Once you removed a node from a multi-node site, you cannot reuse the node in a multi-node environment. There is a description in the following link:
Please check your log and if you find the error “Node disabled (most probable cause is having been unregistered from a cluster). Aborting startup...”, this might have been a cause of the issue.