Just filled in the register form on xing.com. Seconds later I received an email to verify my email and. After clicking on the verify button I was registered and could complete the rest of my profile.
They have a help & contact page, maybe you can try to contact them that way and explain your problem https://help.xing.com/hc/en-us?nwt_nav=footer&segment=b2c
My LinkedIn account and now my Xing account are in "Not interested right now." mode. I was not a heavy user of that platform.
Maybe other people can share there experience with the Xing platform ?