The UI or logs show an error "resource file is missing".
Or an "Object Reference not set to an instance of an object" and the SharePoint logs shows errors for the product.resx file, for example: "Failed to open the language resource keyfile"
Or an error such as:
"Unexpected System.Resources.MissingManifestResourceException: Could not find any resources appropriate for the specified culture or the neutral culture. Make sure "Resources.Bamboo.Conductor.Resources.resources" was correctly embedded or linked into assembly "App_GlobalResources.eb_mozrh" at compile time, or that all the satellite assemblies required are loadable and fully signed."
Or when trying to open the Workflow Conductor Control Panel in Central Administration:
"An error occurred during the compilation of the requested file, or one of its dependencies. The type or namespace name 'Bamboo' does not exist in the namespace 'Resources' (are you missing an assembly reference?)"
This is usually due to the resource files not being not copied to the global resources folder (App_GlobalResources) of the web application after solution deployment. This may happen if the SharePoint Timer service account did not have the appropriate permissions to successfully execute custom job(s) that are scheduled during deployment.
1. Run: stsadm.exe -o copyappbincontent
This command will copy resource files required at the web application level from their locations in the 12, 14. 15. or 16 hive CONFIG folder to the correct location in each web application on the server.
Note: this is not a farm-wide command, it must be invoked on each server in the farm that is running the Web Application Service.
Technet: Copyappbincontent: Stsadm operation (Office SharePoint Server)
2. Try the PowerShell command: Install-SPApplicationContent
Which will also need to be run on every server in the farm that is running the Web Application Service. Note: if the server on the farm that runs the Central Administration service is not running the web application service, you may need to run this command on that server as well, in the case where the Workflow Conductor control panel is throwing this error when you attempt to access it via the Central Administration browser. There may be files that need to be copied by PowerShell or stsadm from the 12 or 14 hive/config to the web application root App_GlobalResources.
Joel's SharePoint Architech blog: STSADM copyappbincontent vs. Install-SPApplicationContent
3. Try reactivating any web application features for the product.
Then be sure that the site collection features are active.
If this does not resolve the issue, then redeploy the solution (by running the product installer).
Note: There may be other files missing besides the resource files. Check for other errors in the UI and ULS Logs