So, my 601 error using my credit card and the paypal rejection afterwards was due to the card being blocked by the credit card provider because of a suspect transaction made just before the FF14 renewal. (The transaction was actually OK, just triggered a verification procedure and a temporary block). My bank was not able to provide this info and I had to contact the credit card provider directly.

In my case, the 601 was not related to SQUARENIX.