SU01 - Reference user "blocked"

Dear experts,

we face the following problem:

In SU01 there is the field "Reference User". If we try to maintain a reference user here, and hit save, the user gets saved, without errors. Then if you open the user again, the reference user field contains only the string "blocked" and not the refuser we wanted.

We have traced and debugged. The table USREFUS gets updated with the correct reference user. That is what we see in ABAP.

Has anybody seen this before?

Thanks and cheers

Boldi