Reported to the Debian BTS by Wolfram Kleff about 7 years ago, still applies.
xrdb fails with spaces in the -I path
$ xrdb "-I/tmp/a b/" .Xresources
cpp: b/: No such file or directory
cpp: warning: '-x c' after last input file has no effect
cpp: no input files
Mass closure: This bug has been untouched for more than six years, and is not
obviously still valid. Please reopen this bug or file a new report if you continue to experience issues with current releases.