Sorry for bothering. I already have the answer:
After the release of Horde 2.1, the isWarning() function was removed from PEAR, resulting in this error when using a PHP/PEAR released after Horde 2.1 was released. Possible solutions are:
- Upgrade to a newer Horde (e.g. CVS version of Horde)
- Remove all references to PEAR::isWarning() from the code manually.
- Downgrade to an older PHP/PEAR version.