NASA would never cancel/delay an EVA
unless one of its intended spacewalkers
was sick as a dog.
Think extreme "space sickness";
vomiting, vertigo, etc... the whole nine yards.

Or....One of the spacesuits for the intended
EVA has a leak, or there's a hazardous form of contamination,
or a faulty mechanical device in the lifesupport
backpack.

I think the ESA crewman sent up by the space shuttle
is the problem behind the delayed spacewalk.
My money is on the likilihood that he's sick...very sick
at this point in time.

If you puke inside your helmet during a spacewalk
you could literally suffocate by clogging your breathing
valves.
Back in 1969, NASA's flight-surgeons were alarmed
by the possibility that Russell Schweikart's spacewalk
during the Apollo 9 mission might prove deadly
because of his extreme bout with space sickness.

I hope the ESA crewman recovers speedily.