Please confirm whether or not above is the issue you're experiencing. If it's not then you'll need to provide more information as to what it was that you were attempting to do before the dialog appeared.
Google translation of error: "Failed to initialize storage of pixel mask. Could not create lock file inside the folder: Lightroom Catalog-v12-2.Ircat data. The process cannot access the folder because it used by another process."
The thread referenced by Ian contains this solution: