unlocking of a disk depending on user?

I have a machine (10.9.4) with an encrypted external disk (FireWire) for Time Machine.

Three users automatically unlock the Time Machine disk when logging in after booting.

However, the fourth user can't seem to unlock the Time Machine disk and I can't think of any reason for this.

Any suggestions?

The three unlocking users all have the same expected password in their keychains, but it is refused when entered by the fourth user.

The Time Machine is a partition of a disk.

When the fourth user can't unlock the Time Machine partition, the disk can be repaired in Disk Utility by this user but the Time Machine partition is greyed out.

I'v tried to find other entries for this subject, but Apple and Google only seem to find situations where no user can unlock a disk; I'm interested in the question why this particular user can't unlock the disk.