Is this any help ?
My message log has this ;
May 6 18:15:00 old-road2 HORDE[3593]: [] imp is not activated [on line 759 of "/home/httpd/html/horde/lib/Horde/Registry.php"]
I know Charlie said to open a bug report, but the only time I have seen this error is when imp is set to inactive in registry.php. By default imp is always set to active in this file. Are you sure you didn't make any changes, or have a custom-template overriding this setting. Check the file /home/httpd/html/horde/config/registry.php and look for the //120AppRegistryImp line, status should say active. If it doesn't, that is probably the issue. You can change it to active to verify that it will fix your problem, then you need to find out how it got changed. Doing a signal-event email-update will re-expand the templates. If imp status is set to inactive, then you know that a template file is doing it.
In fact, if I change the status of imp in the registry.php, I get the exact same error in my log that you do.
Jul 7 18:49:31 dlkfjawww HORDE[4647]: [] imp is not activated [on line 759 of "/home/httpd/html/horde/lib/Horde/Registry.php"]
Here is the error from the web
A fatal error has occurred
imp is not activated
[line 759 of /home/httpd/html/horde/lib/Horde/Registry.php]
I also removed the horde components, re-installed them, then enabled the smeupdates-testing repo and upgraded the components and imp's status is set to enabled.
John