Meaning of the error "Value was not prepared for Update"

[Fran=?ISO-8859-1?B?5w==?=ois Van Lerberghe]

Hello,

Tomorrox I have had error # 8050E : "Field "xxx" : value was not prepared
for Update".
This error occur after another one (# 23503 : "Attempt to insert duplicate
key row in object "xxx" with unique index "zzz"), resulting of an action
where I attemp to add a duplicate value in a field having unique attribute
set. Following this first error, I have tried to update a record in the
table and got this second (# 8050E).

Can someone explain me the exact meaning of that second error and what I can
do to solve/avoid it.

Thank you.

François Van Lerberghe
Rue Thier Monty, 15 A
4570 Marchin
Belgique