I haven't seen that particular error message before, but checking in the .mcw file isn't necessary. It defines things like window positions which isn't important to version control. If you are sharing the repository, everyone should have their own .mcw file.
Edit: found the following in the MPLAB users manual:
Quote:
Do not place the workspace (MCW) or build-state (MCS) files on the repository.
MPLAB IDE needs to be able to write to these files while running.
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It's likely that MPLAB has the files locked from editing and that is causing the error message.