P: There was an unknown error while writing metadat to this photo. Retry?

After I copied all my photo files in Lightroom to a new PC, all the photo files have been marked with an exclamation mark in the upper right corner. When I mouse over the exclamation mark, the message appears: "Error writing metadata". When I click on this mark, I get the following message: "There was an unknown error while writing metadata to this photo. Retry?" Below the message there are two options I can click: "Import settings from disk" and "Retry Metadata Export". I don't understand what Lightroom is trying to tell me, nor what the consequences are of clicking these two options.