Thanks for the help, Al Bina and Opa. As you have quite rightly pointed out, Al Bina, it is now working.
I think I may have found the crux of the problem, after conducting some experimentation with my password. Primarily, it is worth noting, I use long complicated passwords for all my accounts. That being said, the first thing I did was to take the advice from Opa and enter a simple password. This worked first time and I was able to enter the cave.
However, the assumption made by Opa that I must have entered it wrong 3 times is flawed.
Opa wrote:...If it still doesn't work maybe try to make a password you can't enter wrong (e.g. 0000 or aaaa). If it works then you always entered the wrong password, otherwise we get a problem...(Don't forget to change you password back after this try)
Opa
The reason I say this is flawed is because I said, in my origional post...
SmilingOne wrote:...I've also copy and pasted it...
It is nigh on impossible to enter the password incorrectly if the paste function is used.
The next step I took was to shorten the origional password I used. I reduced it to approximately 12 characters. This also worked. I then proceeded to increase the number of characters incrementally by one each time, until it failed. I found that it failed at 16 characters. I tried this 3 times with different passwords.
The next step I took was to simplify the password to just one character repeated 16 times. This also failed. The moment I changed back to 15 characters, it worked again.
To sum up, I feel this highlights either a limitation in password length or a flaw in the encryption algorithm.
SO