When exporting a component as a PNG (and overwriting an existing file with the same name), then editing the component an trying to export it once more, quite often (but not always...) an error occurs ("There was a problem exporting your asset.") The exported file isn't locked or used by any other application. Sometimes shutting down XD and starting again helps, but not always. The only work-around is to rename the asset, exporting it, and then renaming it back to the original name, which obviously is not a proper solution.
We are sorry to hear you are having trouble exporting an asset in XD. I am unable to reproduce the same at my end, may be I am missing something. Would you mind sharing the OS and XD version of your machine? Is it possible for you to share a short video of the issue whenever it happen for better understanding?
In order to reproduce the error, I think one has to drag&drop the exported file (in this case a PNG) back into XD. I can't be 100% sure but it seems to be the case for me.
So, my work process is to edit the asset, export it as PNG, then drag that back into XD next to the asset for comparison of the rendition. Whenever I try export that asset again (e.g. after making some edits) this problem occurs.
This doesn't look usual to me. I suggest getting in touch with the tech support team https://adobe.ly/2WYE62m who can perform a more comprehensive investigation, and log a bug wth the product team if necessary.