Had the exact same issue. Tracked it down to the following:
In your WebSphere Console navigate to Environment -> Shared Libraries
Select the ECMClientSharedLib and verify that the listed jar files are correct.

What I noticed is that the names did not match with the actual file names.

-SP

I installed CMOD 10.5.0.0 on a new Windows server and configured it. I was able to successfully use arsstash.exe to stash the ID and password in a stash file.

I then upgraded the server to 10.5.0.8 and now the arsstash command errors out including when trying to change the configuration using the OnDemand Configurator.

The error displayed is:
ARS1602E The stash file >ARCHIVE.stash< is invalid.