Looked at the url provided by @kieran , because the message alarmed me, too.
There is an update which points to this comment which says that it is a Debian bug. I looked at /usr/bin/ckbcomp and added the following line:
user@system:/usr/bin$ diff -u ckbcomp.orig ckbcomp
--- ckbcomp.orig 2019-10-08 20:21:06.996506838 +0200
+++ ckbcomp 2019-10-08 22:59:39.298948647 +0200
@@ -398,6 +398,7 @@
}
my %xkbsym_table = (
+ 'dead_belowmacron' => 'fe68',
'space' => '0020',
'exclam' => '0021',
'quotedbl' => '0022',
WARNING: Unknown X keysym “dead_belowmacron” gone 