I ran into this Horde error with a client that was trying to add additional identities in Horde groupware 5.2.6 “Preferences for Mail” “Personal Information” but was getting this error message in a red popup in lower right corner:
"The preference 'identities' could not be saved because its data exceeds the maximum allowable size."
This error occured as well when the client tried to edit an existing identity; he got the same error message.
We were able to resolve this by modifying the /usr/local/cpanel/base/horde/config/conf.php file from
$conf['prefs']['maxsize'] = 65535;
$conf['prefs']['maxsize'] = 100000;
which resolved the issue.
Granted, this may not be the best way to address this but there was little to no information out in the wild, wild web about it so… dropping it in here!