Tente isto:
find /home/USER/logfilesError/ -maxdepth 1 -type f -name "xy_*" \
-daystart -mtime -1 -exec grep -Hl "ERROR" "{}" +
De man find
:
-exec command {} +
This variant of the -exec action runs the specified command on
the selected files, but the command line is built by appending
each selected file name at the end; the total number of invoca‐
tions of the command will be much less than the number of
matched files. The command line is built in much the same way
that xargs builds its command lines. Only one instance of '{}'
is allowed within the command. The command is executed in the
starting directory.