"Critical error in configuration file" errors

If you encounter the error, it means the settings stored in the TeamCity Data Directory are in inconsistent state. This can occur after manual change of the files or if newer version of TeamCity starts to report the inconsistencies.
To resolve the issue, you can edit the file noted in the message on the server file system. (make sure to create backup copy of the file before any manual edits). Usually server restart is not necessary for the changes to take effect.

VCS root with id "XXX" does not exist
The build configuration or template reference a VCS root which is not defined in the system.
Remedy actions: Restore the VCS root or create a new VCS root with the id noted or edit the file noted in the message to remove the reference to the VCS root.