An UBLAS patch called: "Remove deprecated inheritance from std::iterator #97" didn't make it into the released version of Boost.
"std::iterator was deprecated in C++17 and removed in C++20. I replaced the inheritance with the 5 equivalent typedefs, even though they're not all used by ublas, for compatibility in case clients depend on them."
If I copy the file latest development version of the boost/numeric/ublas/detail/iterator.hpp into my boost directory it fixes the issue when building with VS 2019 C++20.
The fix was over 2 years ago and should have made it into Boost 1_75.
-Kurt