Indeed, default they'll give you a WIF Compressed private key, starting with K or L, so i wonder what happened since you ended up with 50 letters starting with a 5.
The two things popping to mind:

1) you used the wallet details to get a private key in WIF format, written it down and forgot 1 letter... Maybe even switched some other letters or couldn't read your handwriting
2) you've written down the WIF compressed PK, but forgot the initial K or L AND one extra letter... The odds of the second letter in the WIF compressed pk being 5 is pretty small tough...

I have to ask: is there any chance you might have printed out the page from bitaddress... Chances of mixing up letters and bad handwriting are pretty small on a printed piece of paper.... I know, it's a longshot, but it's certainly worth your effort to try and remember this