That’s unlucky: two unrelated issues on different platforms…
The first issue likely relates to your executablles having been linked at compile-time with a different version of the Qt library than the one that is used at runtime. Have a look at the output of:
ldd ($which mrview)
and compare to the libraries actually linked to at compile time, which you should be able to figure out from the config file – look for the -L flags in the qt_ldflags entry.
Please post both the output of the command above and the contents of your config file so we can take a look.
I can actually reproduce the second issue if I try to run the configure script using the ‘wrong’ version of the python executable. It would be quite difficult to properly explain the reason for what you’re seeing, but it relates to the idiosyncracies of MSYS2 and Windows. There are multiple versions of python installed on your system: one is a Windows-native version (located in the /mingw64/bin folder, part of the mingw-w64-x86_64-python package), the other is a Unix-compatible version (located in /usr/bin, part of the python package). They behave slightly differently, and configure will only run properly using the /usr/bin/python version. So I’ve put in a hack to detect when it’s running with the wrong version and transparently switch to the proper one – this looks like it’s not working any more…
To cut a long story short, try adding .decode(errors='ignore') at the right place in configure, line 154, so it looks like this:
sys.exit (subprocess.call ([ python_cmd.decode(errors='ignore') ] + sys.argv))
This seems to fix the issue on my system. If that works (or doesn’t), please report back, and I’ll commit the change to the main repo.