Hi,
That error is saying that the file you clicked on is missing from your filesystem. It's giving you the location where DSpace expects that file to be found. It thinks it should be located at "/usr/local/dspace/assetstore/15/70/61/157061081078946674319142175541180994295". That location is based on the configuration of "assetstore.dir" in your local.cfg.
While that looks like an obscure filename, DSpace removes the file extension. So, the file should be stored as-is in that location. If it was a PDF, then you should be able to open it in a PDF reader. If it was a Word doc, it should be possible to open in Word, etc.
Unfortunately, it's hard to say what might have happened here. It's possible these files were lost during a recent upgrade. It's possible you have a very active virus scanner which has decided these few files were dangerous and has put them into a quarantine space. Or it's possible these files were accidentally deleted in some other manner. You'd have to investigate why only some files are missing, while it sounds like others are working fine.
To get rid of this error, you'd need to find some way to restore (restore them into that assetstore location) or replace the files (delete the old ones that are throwing errors & re-upload). If you have a recent backup of your DSpace filesystem, hopefully you can simply restore them from that backup.
Tim