syslog error: xfce4-notifyd-theme.rc: unterminated string constant

I have 5 times the following message in my syslog:

org.freedesktop.Notifications[1801]: /home/stefan/.cache/xfce4-notifyd-theme.rc:1: error: scanner: unterminated string constant

searching for /xfce4-notifyd-theme.rc reveals a file with suffix .broken:

locate xfce4-notifyd-theme.rc
/home/stefan/.cache/xfce4-notifyd-theme.rc.broken

The content of that file is pretty brief:

cat $(locate xfce4-notifyd-theme )
include "

Well, yes, indeed, this looks like an unterminated String constant. But where does it come from?

Searching the file on the internet only gives me accidental matches, where people have the same error message in their log, but look for something else.

How do I fix the underlying problem?

Is there a xfce4-notifyd-theme.rc (without postfix .broken) which I can find and place somewhere?