Further investigation:

diff -ru orig/gdm-3.26.2.1/daemon/gdm-session.c gdm-3.26.2.1/daemon/gdm-session.c
--- orig/gdm-3.26.2.1/daemon/gdm-session.c      1970-01-01 01:00:00.000000000 +0100
+++ gdm-3.26.2.1/daemon/gdm-session.c   2019-04-21 10:58:31.211164558 +0200
@@ -2483,6 +2483,10 @@
                                       const char *value)
 {
         g_return_if_fail (key != NULL);
+        if (value == NULL) {
+                g_warning ("XXX gdm: value == NULL for key == %s", key);
+                value = "";
+        }
         g_return_if_fail (value != NULL);
 
         g_hash_table_replace (self->priv->environment,

Yields:

Apr 21 11:23:57 localhost gdm: XXX gdm: value == NULL for key == PATH 
Apr 21 11:23:57 localhost gdm: Child process -1384 was already dead. 
Apr 21 11:23:57 localhost gdm: Child process 1381 was already dead. 
Apr 21 11:23:57 localhost gdm: Unable to kill session worker process 

and endless loop.