Thanks to all of you for your advice. I disabled the reading of metadata during import. Then Calibre was able to import my PDF. I have no way to change the medata in the PDF.
Following kovidgoyal's suggestion, I looked into the maximum file name length for ext4, the filesystem I'm using on my Linux box. It's a local filesystem. The upper limit is 255 ASCII characters. However, my home directory is encrypted using eCryptfs and
that has an upper limit of 143 ASCII characters. The file name as proposed by Calibre has 158 ASCII characters. Mystery solved!