WalterH User has full permissions and delegates configured via the admin ui for the shared user.
To create the rule, I log into the Grommunio webmail with the user which has the full pem+delegates and in the settings I create a rule for the shared user, eg move every mail received from a specific adress to a subfolder.
If I then test the rule, the mail does not get moved to the specified folder and the following entry shows up in the Gromox http log
TWOSTEP ruleproc unsuccessful: Insufficient access rights to perform the operation
And we also set the lda_twostep_ruleproc, like you described
ls -lrt /etc/gromox/exmdb_local.cfg
-rw-r--r-- 1 root root 23 Feb 7 08:42 /etc/gromox/exmdb_local.cfg