Uppercase versus lower case makes a difference. I may have this backwards, but if your WEP key starts with a letter and you use uppercase, the Android thinks this is a text string and will convert the text string to something hexidecimal, rather than using it as straight hexidecimal.
Basically, in one case it thinks the string is a text string to be converted, in the other case it assumes it is a hexidecimal number.
Another suggestion is to change the Wireless LAN security from WEP to WPA with preshared key. WPA is more secure than WEP plus you can choose your own password instead of using the one provided by the router.