Error No-English language Admin Order issue

Hello All,

I have been facing a issue since got to OC 1.5.4.1 - I always get an alert error when adding a product to an admin order:
Im afraid this has to do with with language, I am using Portuguese and all products are in portuguese. Searched it all over and found this a common issue with my colleagues also using it in portuguese - in spite nobody has yet reported it.

Here is the log error:

2012-10-23 2:37:34 - PHP Warning: json_encode() [function.json-encode]: Invalid UTF-8 sequence in argument in /home/storage/0/df/f6/gravis/public_html/b2b/catalog/controller/checkout/manual.php on line 578

Here is the alert message:

SyntaxError: Unexpected token <
OK
Warning: json_encode() [function.json-encode]: Invalid UTF-8 sequence in argument in /home/storage/0/df/f6/gravis/public_html/b2b/catalog/controller/checkout/manual.php on line 578Warning: json_encode() [function.json-encode]: Invalid UTF-8 sequence in argument in /home/storage/0/df/f6/gravis/public_html/b2b/catalog/controller/checkout/manual.php on line 578Warning: json_encode() [function.json-encode]: Invalid UTF-8 sequence in argument in /home/storage/0/df/f6/gravis/public_html/b2b/catalog/controller/checkout/manual.php on line 578{"order_product":[{"product_id":"1054","name":...... continues

I noticed an odd behaviour:

If I go to insert a new order, and just go to product tab - ignoring to fill data in the "PAYMENT DETAILS" or "SHIPPING DETAILS" tabs, when inserting a product - the issue described above occurs.
But if I fill the data in both of the "PAYMENT DETAILS" and "SHIPPING DETAILS" tabs before to add a product - it works ok.

Also when editing an existing order, by adding any product. The same error happens.

Please, may anyone help?

Regards