This occurs because, with the OSS version, the 'Custom Build Dependancies' textbox in the build step is read-only and it constantly contains the “Requires Artifactory Pro.” string, which is being picked up as a custom build dependency.
So, on every build, it is looking for the following build dependency because of this:
http://ArtifactoryServerName:8081/artifactory/api/search/pattern?pattern=Requires Artifactory Pro.
Which will immediately cause this error:
The workaround we found was to edit the xml of the build configuration that uses the Artifactory plugin, which can found under TeamCity Configuration Data folder:
TeamCity_ConfigurationData\config\projects\ProjectName\buildTypes\BuildConfiguration.xml
Locate the following tag from the build step configuration file, and remove the 'Requires Artifactory Pro.' value
param name="org.jfrog.artifactory.selectedDeployableServer.buildDependencies"
This is a temporary workaround until JFrog fix this bug, please note that re-saving the configuration in TeamCity will re-insert this value and cause the build to fail again