[1.5.4.1](Fresh Install) Syntax Error: Unexpected Token < OK

Hi All,
Am getting the attached error on a brand new Opencart 1.5.4.1 install with no mods or changes, when attempting to checkout with a product. The error occurs after clicking continue when typing in the user details / address (guest or registered) (i.e step 2 & 3).
Have searched the forum and attempted to get more details with firebug but it shows the same issue. All previous complaints about this error appear to have been resolved in 1.5.3.1.
Please help, what do I do from here?
The store is located at http://gt86.com.au/shop if you would like to check it out.
Thank You,
Oliver

On second attempt I get a similar error:
SyntaxError: JSON.parse: unexpected character

OK

<b>Notice</b>: Undefined index: firstname in <b>/home/gt86/public_html/shop/catalog/controller/checkout/guest.php</b> on line <b>185</b><b>Notice</b>: Undefined index: lastname in <b>/home/gt86/public_html/shop/catalog/controller/checkout/guest.php</b> on line <b>189</b><b>Notice</b>: Undefined index: email in <b>/home/gt86/public_html/shop/catalog/controller/checkout/guest.php</b> on line <b>193</b><b>Notice</b>: Undefined index: email in <b>/home/gt86/public_html/shop/catalog/controller/checkout/guest.php</b> on line <b>193</b><b>Notice</b>: Undefined index: telephone in <b>/home/gt86/public_html/shop/catalog/controller/checkout/guest.php</b> on line <b>197</b><b>Notice</b>: Undefined index: address_1 in <b>/home/gt86/public_html/shop/catalog/controller/checkout/guest.php</b> on line <b>224</b><b>Notice</b>: Undefined index: city in <b>/home/gt86/public_html/shop/catalog/controller/checkout/guest.php</b> on line <b>228</b><b>Notice</b>: Undefined index: postcode in <b>/home/gt86/public_html/shop/catalog/controller/checkout/guest.php</b> on line <b>237</b>{"error":{"firstname":"First Name must be between 1 and 32 characters!","lastname":"Last Name must be between 1 and 32 characters!","email":"E-Mail Address does not appear to be valid!","telephone":"Telephone must be between 3 and 32 characters!","address_1":"Address 1 must be between 3 and 128 characters!","city":"City must be between 2 and 128 characters!"}}

Also in the error log from admin panel I can see:
2012-12-27 11:19:40 - PHP Notice: Undefined index: firstname in /home/gt86/public_html/shop/catalog/controller/checkout/guest.php on line 185
2012-12-27 11:19:40 - PHP Notice: Undefined index: lastname in /home/gt86/public_html/shop/catalog/controller/checkout/guest.php on line 189
2012-12-27 11:19:40 - PHP Notice: Undefined index: email in /home/gt86/public_html/shop/catalog/controller/checkout/guest.php on line 193
2012-12-27 11:19:40 - PHP Notice: Undefined index: email in /home/gt86/public_html/shop/catalog/controller/checkout/guest.php on line 193
2012-12-27 11:19:40 - PHP Notice: Undefined index: telephone in /home/gt86/public_html/shop/catalog/controller/checkout/guest.php on line 197
2012-12-27 11:19:40 - PHP Notice: Undefined index: address_1 in /home/gt86/public_html/shop/catalog/controller/checkout/guest.php on line 224
2012-12-27 11:19:40 - PHP Notice: Undefined index: city in /home/gt86/public_html/shop/catalog/controller/checkout/guest.php on line 228
2012-12-27 11:19:40 - PHP Notice: Undefined index: postcode in /home/gt86/public_html/shop/catalog/controller/checkout/guest.php on line 237

Hello, i've saw you've resolved this issue
i've same problem, how i can resolve this ?
thank you
bye

giannicasa wrote:Hello, i've saw you've resolved this issue
i've same problem, how i can resolve this ?
thank you
bye

I wold also like to know how to resolve this issue.

Me too !!!!

*please Help !!!