I'm getting a "Permission denied" message whenever I try to run a shell script, even logged in as root. The exact message: "/bin/sh: Bad interpreter - Permission denied". That should never happen if I'm root. Another related problem I'm having is that I'm unable to log into anything other than the login shell, with any user; for example, if I "lock screen" in KDE, when I'm trying to unlock it it asks for my password, but fails even if I provide the correct password. The su command does the same thing - fails even when I provide the correct password. The passwd command is working only for root (it locks up the terminal if I run it as a normal user). No X-based terminal emulator (konsole, xterm, kterm) will work, possibly for the same reason.

Looking at the contents of /var/log/auth.log after yet another failed attempt of using the 'su' command I found the following:
It does really look like I have a problem with the authentication system. But, how do I get rid of it?

Thanks in advance.

Are the shell scripts executable (chmod +x)?

Where you try to execute a script could cause this problem.

Some partitions have the noexec option. Such as /tmp or /var. Any parition that is world writeable should be mounted with the noexec, nodev, and nosuid options.

Also, look at the first line of the scripts. Do they start with the line: #!/bin/bash
or
#!/bin/sh

I saw the exact message when I tried to execute an install script on cdrom. Sometimes prepending 'sh ' or '/bin/bash ' before the command to execute will work.