[CWEB] Validation of PDF output fails

Dear list,

I am trying to understand the error message

    Validating file "common.pdf" for conformance level pdf1.5
    The value of the key Last is 1274 0 R but must be 1098 0 R.
    The document does not conform to the requested standard.

issued by the 3-Heights PDF Validator Online Tool 
(https://www.pdf-online.com/osa/validate.aspx) for file common.pdf.

common.pdf is created from the CWEB sources 
(https://github.com/ascherer/cweb) with

    touch *.c
    make all
    ./cweave common
    pdftex common

The above values for 'Last' are for CWEB version 3.64c; different values 
occur for CWEB 4.8. Similar errors occur for ctangle.pdf and cweave.pdf, 
but not for cwebman.pdf.

My best guess is that the PDF macros in cwebmac.tex for the pdftex 
engine are faulty.

Please help,
Andreas