citserver fails to start, saying "Database contains compressed data, but this citserver was built without compression support."
This error can occur when you upgrade Citadel or attempt to port your database to a new host, when the old host has ZLib and the new host does not. Since some of your database contains compressed data, the new host will not be able to understand it.
This is easily corrected by installing zlib on your new host and recompiling Citadel. If you are using pre-built packages, this error will never occur, because the packages declare zlib as a dependency. If, however, you built Citadel from source code, you will need to install zlib (either from its source or by installing an operating system package such as zlib-dev or zlib-devel) and then “make clean” and “configure” again so that the Citadel system is built with compression support.
If you installed Citadel using Easy Install, you may need to force a rebuild of your sources. Please see this article: How can I force the Easy Install system to reinstall the programs? for information on how to do this.