Does this help?
Those CPU masks become quite weird with --cpu-affinity. And in the end, it's actually bound to CPUs 0-31.
Interesting, the errors occurred even with a default affinity. I looked deeper and discovered the mask
on Windows is only 32 bits as you suspected. I don't know how it's supposed to support 64 CPUs
with only a 32 bit mask.
With groups enabled the mask is of type KAFFINITY and a member of a struct that also include group info.
KAFFINITY was described as a bit mask to set affinity of "one or more" cpus in a group but doesn't specify
a maximum or further define KAFFINITY.
Maybe you should enable CPU-groups, but cpuminer-opt needs a custom build to use it.