Having trouble paying (error 401)

[Enkidoh]

That was a very common error that affected a lot of players, but unfortunately, it seems that error code hid a multitude of sins and thus trying to work out exactly what has gone wrong is frustratingly difficult.

So much so, there was a very large official thread in the Technical Support section, but curiously it's no longer stickied (but luckily is still on the first page). It can be found here.

The main causes though seem to be:

Mistyped card info.
The browser you're using to access the Mogstation has a security function that blocks 'scripts' from working (which is necessary for the Mogstation to verify your card details as valid)
Lack of funds in the account if it's a debit card and thus the bank is automatically flagging the card as invalid
The card vendor or bank that issued your card is falsely flagging SE's attempts to verify the card as fraud

These are just some of the common causes, so it basically becomes a process of elimination to finding out what has gone wrong and how to fix it.