I also engaged the same issue and the problem had to do with corrupted files of Windows OS. More specifically I had installed and uninstalled Anaconda distro and, as result of these procedures, cmd.exe stopped working. After solving the aforementioned issue (https://stackoverflow.com/questions/66335300/cmd-crashes-with-exit-code-1-after-uninstalling-anaconda), by making cmd.exe to work, I was also able to run again Unstable Flow Analysis at HEC-RAS 6.3.
I suppose that HEC-RAS makes calls to cmd.exe for executing specific subprograms and this malfunction of cmd.exe did not let it operate as it should.