Since a while I obtain an error message trying to compile gretl-ref.pdf. The concrete error message is:

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! Text line contains an invalid character.
l.86 ^^@
        ^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^...

?
! Emergency stop.
l.86 ^^@
        ^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^...

!  ==> Fatal error occurred, no output PDF file produced!
Transcript written on gretl-ref.log.
make[2]: *** [gretl-ref.pdf] Fehler 1
make[2]: *** Warte auf noch nicht beendete Prozesse...

! Text line contains an invalid character.
l.86 ^^@^^@
           ^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^@^^...
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