[Exim] Process failed (69) when writing error message

Hello,
I just noticed that my server (I tried Exim 3.21 and 3.22) cannot
deliver bounces but gets a "broken pipe" error:

Failed addresses:
test@???: no retry items
Deferred addresses:
end of retry processing
sending error message to: marc@???
writing data block fd=7 size=359 timeout=0
writing error 32: Datenuebergabe unterbrochen (broken pipe)
Writing spool header file
Size of headers = 319
LOG: 0 MAIN
Process failed (69) when writing error message to marc@??? (frozen)
end delivery of 14PD3B-0003qa-00

What could be the reason for this strange behaviour?

Thanks,
Marc