Update
Problem was caused by the PATH variable. Unfortunately, there was sourced my $HOME/.local/lib directory as well and it contained an outdated numpy package, which caused the problem. After removing, error disappeared.
However, Next I had problem with vtk. As libraries used by conda version of PyQt5 were outdated, the iris driver failed to load. This was simply solved by redirecting the /anaconda3/envs/mfix-21.1.4/lib/libstdc++.so.6 to the system version.
Now it seems that everything is ok.