I don't have a log, but consider on dest the path some/path/filename.ext.
b2 sync --threads=1 --max-download-streams-per-file 1 --exclude-regex '(.*[.]tmp)' --incremental-mode $src $dest
This will create an empty regular file some/path, so on Linux some/path/filename.ext errors.
A workaround is to exclude trailing /:
b2 sync --threads=1 --max-download-streams-per-file 1 --exclude-regex '(.*[.]tmp)|(.*/$)' --incremental-mode $src $dest
Note that my /tmp is mounted noexec, so I'm running like:
TMPDIR=~/tmp b2 version
b2 command line tool, version 4.6.0 (b2sdk version 2.10.3)