[imp] DB Error: constraint violation when linking an attachment

[Manuel Schwab]

Hello list,

IMP 4.3.4
Horde 3.3.3

When I create a new message in IMP and I add an attachment to the 
message the file is uploaded without any problem.
But when I set "link attachment ("Anhänge verlinken" in German") to 
"yes" before sending the message I get the error:

DB Error: constraint violation

and the message is not sent.

Without the option "link attachment" the file is sent.

Is this a problem because of a wrong configuration from me?

Any help is appreciated.

Thanks.

Manuel